Can there be planets, stars and galaxies made of dark matter or antimatter?

Can there be planets, stars and galaxies made of dark matter or antimatter?

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We know that the universe has more dark and anti matter as compared to normal matter. Can there be dark matter galaxies or antimatter galaxies?

Dark matter galaxies are possible but very speculative. On a theoretical level, they are hard to form because dark matter interacts only gravitationally (see Anders Sandberg's answer), which makes it hard to lose energy and become bound structures. On an observational level, they would be hard to detect. Gravitational lensing can do something, but since one cannot actually see the galaxy, it's also hard to say where the dark galaxy is -- if there is one at all.

Still, people have studied the idea, so it's not impossible.

Antimatter galaxies: At some level the idea that there are antimatter galaxies out here is appealing. First it can solve the baryon asymmetry problem at a stroke. It's also the case that an antimatter star would shine. From long distance, it would also be practically indistinguishable from a "normal" star.

However, there are strong reasons to believe that there are no antimatter galaxies. That's because antimatter annihilates with normal matter, which leaves experimental signatures. If any part of the Earth were made of antimatter, it would immediately vanish in a flash, so we can be sure that the Earth is mostly matter. Similarly, if the Sun were made of antimatter, we would be quickly annihilated (thanks to the antimatter solar wind radiating from the anti-Sun), so we can be sure the Sun is also mostly matter. Similar arguments allow us to conclude that the Milky Way is almost entirely matter, the Local Group is almost entirely matter, etc, all the way up to the largest structures in the sky.

If antimatter galaxies exist, they are probably outside our observable universe, at which point some will argue it's no longer science.

Probably not. Dark matter should really be called "transparent matter" since it does not interact with light. This has an important consequence: it is hard for dark matter - whatever it is - to lose energy by radiating. This is why normal matter can form clouds that accrete into dense regions that in turn become galaxies and stars: energy is radiated away. But dark matter cannot do this as far as we know, so instead it forms large diffuse "halos" that surround galaxies.

Antimatter is completely different from dark matter. For some reason (important research topic) there is far more normal matter than antimatter in the universe, and all primordial antimatter is likely to have reacted with the matter in the early eras. Hence there are not going to be enough of it to form antiplanets, stars or galaxies.

Gravitational lensing observations suggest that there is a large mass of dark matter on either side of the bullet cluster, which is actually one of the major pieces of evidence that dark matter does indeed exist. This dark matter essentially "left behind" the majority of the normal matter in the galaxies it was with as two galaxy clusters collided and most of the normal matter in them got tangled up in the middle. These globs of dark matter with little normal matter probably could, if you like, be considered (in a non-technical sense) to be dark matter galaxies. They're not 100% dark matter, as most of the galaxies' stars also went with them, but they are, at least as I understand it, more dark matter than not.

This isn't the only such object; a similar collision between galaxy clusters produced the object MACS J0025.4-1222, which also consists of several galaxies worth of dust and gas stripped of their dark matter with a pile of dark matter and stars on either side.

Hubble data confirms galaxies lacking dark matter

NGC1052-DF2. Credit: NASA, ESA, Z. Shen and P. van Dokkum (Yale University), and S. Danieli (Institute for Advanced Study)

The most accurate distance measurement yet of ultra-diffuse galaxy (UDG) NGC1052-DF2 (DF2) confirms beyond any shadow of a doubt that it is lacking in dark matter. The newly measured distance of 22.1 +/-1.2 megaparsecs was obtained by an international team of researchers led by Zili Shen and Pieter van Dokkum of Yale University and Shany Danieli, a NASA Hubble Fellow at the Institute for Advanced Study.

"Determining an accurate distance to DF2 has been key in supporting our earlier results," stated Danieli. "The new measurement reported in this study has crucial implications for estimating the physical properties of the galaxy, thus confirming its lack of dark matter."

The results, published in Astrophysical Journal Letters on June 9, 2021, are based on 40 orbits of NASA's Hubble Space Telescope, with imaging by the Advanced Camera for Surveys and a 'tip of the red giant branch' (TRGB) analysis, the gold standard for such refined measurements. In 2019, the team published results measuring the distance to neighboring UDG NGC1052-DF4 (DF4) based on 12 Hubble orbits and TRGB analysis, which provided compelling evidence of missing dark matter. This preferred method expands on the team's 2018 studies that relied on "surface brightness fluctuations" to gage distance. Both galaxies were discovered with the Dragonfly Telephoto Array at the New Mexico Skies observatory.

"We went out on a limb with our initial Hubble observations of this galaxy in 2018," van Dokkum said. "I think people were right to question it because it's such an unusual result. It would be nice if there were a simple explanation, like a wrong distance. But I think it's more fun and more interesting if it actually is a weird galaxy."

In addition to confirming earlier distance findings, the Hubble results indicated that the galaxies were located slightly farther away than previously thought, strengthening the case that they contain little to no dark matter. If DF2 were closer to Earth, as some astronomers claim, it would be intrinsically fainter and less massive, and the galaxy would need dark matter to account for the observed effects of the total mass.

NGC1052-DF2. Credit: NASA, ESA, Z. Shen and P. van Dokkum (Yale University), and S. Danieli (Institute for Advanced Study)

Dark matter is widely considered to be an essential ingredient of galaxies, but this study lends further evidence that its presence may not be inevitable. While dark matter has yet to be directly observed, its gravitational influence is like a glue that holds galaxies together and governs the motion of visible matter. In the case of DF2 and DF4, researchers were able to account for the motion of stars based on stellar mass alone, suggesting a lack or absence of dark matter. Ironically, the detection of galaxies deficient in dark matter will likely help to reveal its puzzling nature and provide new insights into galactic evolution.

While DF2 and DF4 are both comparable in size to the Milky Way galaxy, their total masses are only about one percent of the Milky Way's mass. These ultra-diffuse galaxies were also found to have a large population of especially luminous globular clusters.

This research has generated a great deal of scholarly interest, as well as energetic debate among proponents of alternative theories to dark matter, such as modified newtonian dynamics (MOND). However, with the team's most recent findings—including the relative distances of the two UDGs to NGC1052—such alternative theories seem less likely. Additionally, there is now little uncertainty in the team's distance measurements given the use of the TRGB method. Based on fundamental physics, this method depends on the observation of red giant stars that emit a flash after burning through their helium supply that always happens at the same brightness.

"There's a saying that extraordinary claims require extraordinary evidence, and the new distance measurement strongly supports our previous finding that DF2 is missing dark matter," stated Shen. "Now it's time to move beyond the distance debate and focus on how such galaxies came to exist."

Moving forward, researchers will continue to hunt for more of these oddball galaxies, while considering a number of questions such as: How are UDGs formed? What do they tell us about standard cosmological models? How common are these galaxies, and what other unique properties do they have? It will take uncovering many more dark-matter-lacking galaxies to resolve these mysteries and the ultimate question of what dark matter really is.

Stars made of antimatter could be lurking in our galaxy

Antimatter is the strange, evil twin of regular matter, and it’s thought to have been mostly banished from our universe. But could it still be lurking out there in large clumps, even as stars? Astronomers have now identified a few signals that could be evidence of these “anti-stars,” and calculated how many of them might be hiding in our own galaxy.

As sci-fi as it sounds, antimatter is very real. Simply put, it’s exactly the same as ordinary (or baryonic) matter, except that it has the opposite charge. That means that when particles of matter and antimatter meet, the two annihilate each other in a burst of energy.

According to our best models for the universe, matter and antimatter should have been created in equal amounts in the Big Bang, but today, matter seems to dominate the cosmos. Antimatter is only produced in trace amounts, in instruments like the Large Hadron Collider or through natural processes like lightning, hurricanes, cosmic ray interactions, radioactive decay, or plasma jets from neutron stars and black holes.

So where did all the antimatter go? It seems that it’s almost entirely been wiped out from contact with regular matter – and we were just lucky that there was extra matter left over, otherwise the universe would be a very empty place.

But perhaps the ratio isn’t quite as skewed as we thought. Theoretically, there’s no reason antimatter shouldn’t be able to form stars and galaxies, planets and even life, as long as there was no regular matter nearby to destroy it. It’s an intriguing possibility, but one that’s extremely difficult to validate – after all, anti-stars would shine just like regular ones.

However, they may reveal themselves in other ways. Since it would be pretty difficult for anti-stars to wind up in a region of space completely devoid of regular matter, scientists could potentially spot these impostors through flashes of gamma rays, given off from the annihilation of rogue matter particles that wander too close.

The positions of the anti-star candidate gamma ray signals, overlaid on the Milky Way

And that’s just what astronomers have hunted for in a new study. The team analyzed 10 years’ worth of data from the Fermi Space Telescope, examining 5,787 gamma ray sources for those that could be anti-stars. Lots of other objects also give off gamma rays though, so the researchers focused on those that came from a single point, and had a light spectrum similar to what would be expected from matter-antimatter annihilation.

Sure enough, among those thousands of sources, the team found 14 that fit the bill. That doesn’t mean they are anti-stars, of course – the team acknowledges that it’s far more likely that they’re more well-known gamma ray emitters like pulsars or black holes. But the possibility is there, at least.

From that, the team extrapolated to arrive at an estimate of how many anti-stars there might reasonably be in our galaxy. They found that if anti-stars are distributed like regular stars, and if they don’t have any differences besides charge (something that antimatter studies are still investigating) then we’re looking at around one anti-star for every 300,000 normal stars. Primordial anti-stars might also tend to evade notice by hanging out in the huge, sparse halo around the galaxy too, the team says.

It’s an intriguing idea, and one that will need further study to search for more evidence.

Could Some Distant Galaxies Be Made Of Antimatter?

In astronomy we study distant galaxies by the light they emit. Just as the stars of a galaxy glow bright from the heat of their fusing cores, so too does much of the gas and dust at different wavelengths. The pattern of wavelengths we observe tells us much about a galaxy, because atoms and molecules emit specific patterns of light. Their optical fingerprint tells us the chemical composition of stars and galaxies, among other things. It's generally thought that distant galaxies are made of matter, just like our own solar system, but recently it's been demonstrated that anti-hydrogen emits the same type of light as regular hydrogen. In principle, a galaxy of antimatter would emit the same type of light as a similar galaxy of matter, so how do we know that a distant galaxy really is made of matter?

The basic difference between matter and antimatter is charge. Atoms of matter are made of positively charged nuclei surrounded by negatively charged electrons, while antimatter consists of negatively charged nuclei surrounded by positively charged positrons (anti-electrons). In all of our interactions, both in the lab and when we've sent probes to other planets, things are made of matter. So we can assume that most of the things we see in the Universe are also made of matter.

However, when we create matter from energy in the lab, it is always produced in pairs. We can, for example, create protons in a particle accelerator, but we also create an equal amount of anti-protons. This is due to a symmetry between matter and antimatter, and it leads to a problem in cosmology. In the early Universe, when the intense energy of the big bang produced matter, did it also produce an equal amount of antimatter? If so, why do we see a Universe that's dominated by matter? The most common explanation is that there is a subtle difference between matter and antimatter. This difference wouldn't normally be noticed, but on a cosmic scale it means the big bang produced more matter than antimatter.

But suppose the Universe does have an equal amount of matter and antimatter, but early on the two were clumped into different regions. While our corner of the Universe is dominated by matter, perhaps there are distant galaxies or clusters of galaxies that are dominated by antimatter. Since the spectrum of light from matter and antimatter is the same, a distant antimatter galaxy would look the same to us as if it were made of matter. Since we can't travel to distant galaxies directly to prove their made of matter, how can we be sure antimatter galaxies don't exist?

One clue comes from the way matter and antimatter interact. Although both behave much the same on their own, when matter and antimatter collide they can annihilate each other to produce intense gamma rays. Although the vast regions between galaxies are mostly empty, they aren't complete vacuums. Small amounts of gas and dust drift between galaxies, creating an intergalactic wind. If a galaxy were made of antimatter, any small amounts of matter from the intergalactic wind would annihilate with antimatter on the outer edges of the galaxy and produce gamma rays. If some galaxies were matter and some antimatter, we would expect to see gamma ray emissions in the regions between them. We don't see that. Not between our Milky Way and other nearby galaxies, and not between more distant galaxies. Since our region of space is dominated by matter, we can reasonably assume that other galaxies are matter as well.

It's still possible that our visible universe just happens to be matter dominated. There may be other regions beyond the visible universe that are dominated by antimatter, and its simply too far away for us to see. That's one possible solution to the matter-antimatter cosmology problem. But that would be an odd coincidence given the scale of the visible universe.

So there might be distant antimatter galaxies in the Universe, but we can be confident that the galaxies we do see are made of matter just like us.

Are There Antimatter Galaxies?

One of the biggest mysteries in astronomy is the question, where did all the antimatter go? Shortly after the Big Bang, there were almost equal amounts of matter and antimatter. I say almost, because there was a tiny bit more matter, really. And after the matter and antimatter crashed into each other and annihilated, we were left with all the matter we see in the Universe.

You, and everything you know is just a mathematical remainder, left over from the great division of the Universe’s first day.

We did a whole article on this mystery, so I won’t get into it too deeply.

But is it possible that the antimatter didn’t actually go anywhere? That it’s all still there in the Universe, floating in galaxies of antimatter, made up of antimatter stars, surrounded by antimatter planets, filled with antimatter aliens?

Aliens who are friendly and wonderful in every way, except if we hugged, we’d annihilate and detonate with the energy of gigatons of TNT. It’s sort of tragic, really.

If those antimatter galaxies are out there, could we detect them and communicate with those aliens?

First, a quick recap on antimatter.

Antimatter is just like matter in almost every way. Atoms have same atomic mass and the exact same properties, it’s just that all the charges are reversed. Antielectrons have a positive charge, antihydrogen is made up of an antiproton and a positron (instead of a proton and an electron).

It turns out this reversal of charge causes regular matter and antimatter to annihilate when they make contact, converting all their mass into pure energy when they come together.

We can make antimatter in the laboratory with particle accelerators, and there are natural sources of the stuff. For example, when a neutron star or black hole consumes a star, it can spew out particles of antimatter.

In fact, astronomers have detected vast clouds of antimatter in our own Milky Way, generated largely by black holes and neutron stars grinding up their binary companions.

Wyoming Milky Way set. Credit and copyright: Randy Halverson.

But our galaxy is mostly made up of regular matter. This antimatter is detectable because it’s constantly crashing into the gas, dust, planets and stars that make up the Milky Way. This stuff can’t get very far without hitting anything and detonating.

Now, back to the original question, could you have an entire galaxy made up of antimatter? In theory, yes, it would behave just like a regular galaxy. As long as there wasn’t any matter to interact with.

And that’s the problem. If these galaxies were out there, we’d see them interacting with the regular matter surrounding them. They would be blasting out radiation from all the annihilations from all the regular matter gas, dust, stars and planets wandering into an antimatter minefield.

Astronomers don’t see this as far as they look, just the regular, quiet and calm matter out to the edge of the observable Universe.

That doesn’t make it completely impossible, though, there could be galaxies of antimatter as long as they’re completely cut off from regular matter.

But even those would be detectable by the supernova explosions within them. A normally matter supernova generates fast moving neutrinos, while an antimatter supernova would generate a different collection of particles. This would be a dead giveaway.

There’s one open question about antimatter that might make this a deeper mystery. Scientists think that antimatter, like regular matter, has regular gravity. Matter and antimatter galaxies would be attracted to each other, encouraging annihilation.

But scientists don’t actually know this definitively yet. It’s possible that antimatter has antigravity. An atom of antihydrogen might actually fall upwards, accelerating away from the center of the Earth.

The ALPHA experiment, one of five experiments that are studying antimatter at CERN Credit: Maximilien Brice/CERN

Physicists at CERN have been generating antimatter particles, and trying to detect if they’re falling downward or up.

If that was the case, then antimatter galaxies might be able to repel particles of regular matter, preventing the annihilation, and the detection.

If you were hoping there are antimatter lurking out there, hoarding all that precious future energy, I’m sorry to say, but astronomers have looked and they haven’t found it. Just like the socks in your dryer, we may never discover where it all went.

Can there be planets, stars and galaxies made of dark matter or antimatter? - Astronomy

Are there stars or galaxies made entirely of dark matter, instead of the normal matter like the Sun and Earth are made of?

Current models characterize dark matter as some sort of particle which interacts very weakly electromagnetically (which is why it doesn't emit light, and why it is called dark matter) and primarily interacts gravitationally. In the formation of galaxies, it is the overdensities of dark matter that gravitationally attracts into higher density clumps first, which then provides the gravitational force for ordinary matter to clump up within it. However, in order for ordinary matter to collapse enough to form stars, it must lose a great deal of energy and angular momentum. Ordinary matter can lose energy and angular momentum through radiation and collisions between the particles, but dark matter particles don't do this because of their weak electromagnetic interactions.

As a result, it is extremely unlikley there are very dense objects like stars made out of entirely (or even mostly) dark matter.

"Dark galaxies" are a bit more complicated. Galaxies that are visible to us mark the places where dark matter is concentrated the normal matter that we see as stars and gas is located in the high density central regions of these halos of dark matter. Theoretically there are conditions where the smallest dark matter halos are too small to attract enough normal matter for stars to form (or the few stars that do form have enough energy to push out the rest of the normal matter out via radiation or supernovae), in which case the dark matter halo can be said to be a "dark galaxy". But if a "dark galaxy" emits no light, it would be very tricky for us astronomers to ever detect it! One possibility would be detection via gravitational lensing, since that only depends on the dark galaxy's mass to gravitationally warp the path of some background object's light. But strong lensing detections need lots of matter (or very concentrated matter, depending on the type of lensing you are interested in), which probably means that clumps of dark matter that are small and diffuse enough to remain "dark galaxies" won't produce much lensing. So while "dark galaxies" could maybe exist, no one has found one yet!

This page was last updated by Chelsea Sharon on July 18, 2015

About the Author

Jagadheep D. Pandian

Jagadheep built a new receiver for the Arecibo radio telescope that works between 6 and 8 GHz. He studies 6.7 GHz methanol masers in our Galaxy. These masers occur at sites where massive stars are being born. He got his Ph.D from Cornell in January 2007 and was a postdoctoral fellow at the Max Planck Insitute for Radio Astronomy in Germany. After that, he worked at the Institute for Astronomy at the University of Hawaii as the Submillimeter Postdoctoral Fellow. Jagadheep is currently at the Indian Institute of Space Scence and Technology.

Stars made of antimatter could lurk in our galaxy

Fourteen celestial sources of gamma rays look like they could be stars made of antimatter. Each potential antistar is a colored dot on this map of the Milky Way. Greener dots are brighter sources bluer dots are dimmer.

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All known stars are made of ordinary matter. But astronomers haven’t completely ruled out that some could be made of antimatter.

Antimatter is the oppositely charged alter-ego of normal matter. For instance, electrons have antimatter twins called positrons. Where electrons have negative electric charge, positrons have positive charge. Physicists think the universe was born with equal amounts of matter and antimatter. Now the cosmos appears to have almost no antimatter.

Space-station data have recently cast doubt on this idea of a practically antimatter-free universe. One instrument might have seen bits of antihelium atoms in space. Those observations have to be confirmed. But if they are, that antimatter could have been shed by antimatter stars. That is, antistars.

Explainer: What are black holes?

Intrigued by this idea, some researchers went hunting for potential antistars. The team knew that matter and antimatter annihilate each other when they meet. That could happen when normal matter from interstellar space falls onto an antistar. This type of particle annihilation gives off gamma rays with certain wavelengths. So the team looked for those wavelengths in data from the Fermi Gamma-ray Space Telescope.

Fourteen spots in the sky gave off the gamma rays expected from matter-antimatter annihilation events. Those spots did not look like other known gamma-ray sources — such as spinning neutron stars or black holes. That was further evidence that the sources could be antistars. Researchers reported their find online April 20 in Physical Review D.

Rare — or possibly hiding?

The team then estimated how many antistars could exist near our solar system. Those estimates depended on where antistars would most likely be found, if they truly existed.

Any in the disk of our galaxy would be surrounded by lots of normal matter. That could cause them to emit lots of gamma rays. So they should be easy to spot. But the researchers only found 14 candidates.

That implies that antistars are rare. How rare? Perhaps only one antistar would exist for every 400,000 normal stars.

Understanding light and other forms of energy on the move

Antistars could exist, however, outside the Milky Way’s disk. There, they would have less chance to interact with normal matter. They also should emit fewer gamma rays in this more isolated environment. And that would make them harder to find. But in that scenario, one antistar could lurk among every 10 normal stars.

Antistars are still only hypothetical. In fact, proving any object is an antistar could be nearly impossible. Why? Because antistars are expected to look almost identical to normal stars, explains Simon Dupourqué. He’s an astrophysicist in Toulouse, France. He works at the Institute of Research in Astrophysics and Planetology.

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It would be much easier to prove the candidates found so far are not antistars, he says. Astronomers could watch how gamma rays from the candidates change over time. Those changes might hint at whether these objects are really spinning neutron stars. Other types of radiation from the objects might point to their actually being black holes.

If antistars exist, “that would be a major blow” for our understanding of the universe. So concludes Pierre Salati, who wasn’t involved in the work. This astrophysicist works at the Annecy-le-Vieux Laboratory of Theoretical Physics in France. Seeing antistars would mean that not all of the universe’s antimatter was lost. Instead, some would have survived in isolated pockets of space.

But antistars probably could not make up for all the universe’s missing antimatter. At least, that’s what Julian Heeck thinks. A physicist at the University of Virginia in Charlottesville, he too did not take part in the study. And, he adds, “you would still need an explanation for why matter overall dominates over antimatter.”

Power Words

antimatter: Molecules formed by atoms consisting of antiprotons, antineutrons, and positrons.

astronomer: A scientist who works in the field of research that deals with celestial objects, space and the physical universe.

astrophysics: An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.

atom: The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

black hole: A region of space having a gravitational field so intense that no matter or radiation (including light) can escape.

cosmos: (adj. cosmic) A term that refers to the universe and everything within it.

disk: A round, flat and usually fairly thin object. (in astronomy) A rotating cloudlike collection of gases, dust or both from which planets may form. Or the structure of certain large rotating bodies in the cosmos, including spiral galaxies such as our Milky Way.

electric charge: The physical property responsible for electric force it can be negative or positive.

electron: A negatively charged particle, usually found orbiting the outer regions of an atom also, the carrier of electricity within solids.

galaxy: A group of stars — and usually dark matter — all held together by gravity. Giant galaxies, such as the Milky Way, often have more than 100 billion stars. The dimmest galaxies may have just a few thousand. Some galaxies also have gas and dust from which they make new stars.

gamma rays: High-energy radiation often generated by processes in and around exploding stars. Gamma rays are the most energetic form of light.

hypothetical: An adjective that described some hypothesis, or proposed explanation for a phenomenon. In science, a hypothesis is an idea that must be rigorously tested before it is accepted or rejected.

interstellar: Between stars.

matter: Something that occupies space and has mass. Anything on Earth with matter will have a property described as "weight."

Milky Way: The galaxy in which Earth’s solar system resides.

neutron star: The very dense corpse of what had once been a massive star. As the star died in a supernova explosion, its outer layers shot out into space. Its core then collapsed under its intense gravity, causing protons and electrons in its atoms to fuse into neutrons (hence the star’s name). A single teaspoonful of a neutron star, on Earth, would weigh more than a billion tons.

particle: A minute amount of something.

physical: (adj.) A term for things that exist in the real world, as opposed to in memories or the imagination. It can also refer to properties of materials that are due to their size and non-chemical interactions (such as when one block slams with force into another).

physics: The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study that emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in such areas is known as a physicist.

positron: A subatomic particle with the mass of an electron, but a positive electrical charge. It is the antimatter counterpart to the electron. So when electrons and positrons collide, they annihilate each other, releasing energy.

scenario: A possible (or likely) sequence of events and how they might play out.

solar: Having to do with the sun or the radiation it emits. It comes from sol, Latin for sun.

solar system: The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.

star: The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become hot enough, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

telescope: Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.

theoretical physics: A branch of physics that uses mathematical models to understand the nature and properties of matter and energy. A scientist who works in that field is known as a theoretical physicist .

universe: The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).

wavelength: The distance between one peak and the next in a series of waves, or the distance between one trough and the next. It’s also one of the “yardsticks” used to measure radiation. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.


Journal:​ ​​ S. Dupourqué, L. Tibaldo and P. von Ballmoos. Constraints on the antistar fraction in the solar system neighborhood from the 10-year Fermi Large Area Telescope gamma-ray source catalog. Physical Review D. Published online April 20, 2021. doi: 10.1103/PhysRevD.103.083016.

About Maria Temming

Maria Temming is the staff reporter for physical sciences, covering everything from chemistry to computer science and cosmology. She has bachelor's degrees in physics and English, and a master's in science writing.

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Is dark matter real, or have we misunderstood gravity?

For many years now, astronomers and physicists have been in a conflict. Is the mysterious dark matter that we observe deep in the Universe real, or is what we see the result of subtle deviations from the laws of gravity as we know them? In 2016, Dutch physicist Erik Verlinde proposed a theory of the second kind: emergent gravity. New research, published in Astronomy & Astrophysics this week, pushes the limits of dark matter observations to the unknown outer regions of galaxies, and in doing so re-evaluates several dark matter models and alternative theories of gravity. Measurements of the gravity of 259,000 isolated galaxies show a very close relation between the contributions of dark matter and those of ordinary matter, as predicted in Verlinde’s theory of emergent gravity and an alternative model called Modified Newtonian Dynamics. However, the results also appear to agree with a computer simulation of the Universe that assumes that dark matter is ‘real stuff’.

In the centre of the image the elliptical galaxy NGC5982, and to the right the spiral galaxy NGC5985. These two types of galaxies turn out to behave very differently when it comes to the extra gravity – and therefore possibly the dark matter – in their outer regions. Credit: Bart Delsaert (

The new research was carried out by an international team of astronomers, led by Margot Brouwer (RUG and UvA). Further important roles were played by Kyle Oman (RUG and Durham University) and Edwin Valentijn (RUG). In 2016, Brouwer also performed a first test of Verlinde’s ideas this time, Verlinde himself also joined the research team.

So far, dark matter has never been observed directly—hence the name. What astronomers observe in the night sky are the consequences of matter that is potentially present: bending of starlight, stars that move faster than expected, and even effects on the motion of entire galaxies. Without a doubt all of these effects are caused by gravity, but the question is: are we truly observing additional gravity, caused by invisible matter, or are the laws of gravity themselves the thing that we haven’t fully understood yet?

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To answer this question, the new research uses a similar method to the one used in the original test in 2016. Brouwer and her colleagues make use of an ongoing series of photographic measurements that started ten years ago: the KiloDegree Survey (KiDS), performed using ESO’s VLT Survey Telescope in Chili. In these observations one measures how starlight from far away galaxies is bent by gravity on its way to our telescopes. Whereas in 2016 the measurements of such ‘lens effects’ only covered an area of about 180 square degrees on the night sky, in the mean time this has been extended to about 1000 square degrees—allowing the researchers to measure the distribution of gravity in around a million different galaxies.

Brouwer and her colleagues selected over 259,000 isolated galaxies, for which they were able to measure the so-called ‘Radial Acceleration Relation’ (RAR). This RAR compares the amount of gravity expected based on the visible matter in the galaxy, to the amount of gravity that is actually present—in other words: the result shows how much ‘extra’ gravity there is, in addition to that due to normal matter. Until now, the amount of extra gravity had only been determined in the outer regions of galaxies by observing the motions of stars, and in a region about five times larger by measuring the rotational velocity of cold gas. Using the lensing effects of gravity, the researchers were now able to determine the RAR at gravitational strengths which were one hundred times smaller, allowing them to penetrate much deeper into the regions far outside the individual galaxies.

This made it possible to measure the extra gravity extremely precisely—but is this gravity the result of invisible dark matter, or do we need to improve our understanding of gravity itself? Author Kyle Oman indicates that the assumption of ‘real stuff’ at least partially appears to work: “In our research, we compare the measurements to four different theoretical models: two that assume the existence of dark matter and form the base of computer simulations of our universe, and two that modify the laws of gravity—Erik Verlinde’s model of emergent gravity and the so-called ‘Modified Newtonian Dynamics’ or MOND. One of the two dark matter simulations, MICE, makes predictions that match our measurements very nicely. It came as a surprise to us that the other simulation, BAHAMAS, led to very different predictions. That the predictions of the two models differed at all was already surprising, since the models are so similar. But moreover, we would have expected that if a difference would show up, BAHAMAS was going to perform best. BAHAMAS is a much more detailed model than MICE, approaching our current understanding of how galaxies form in a universe with dark matter much closer. Still, MICE performs better if we compare its predictions to our measurements. In the future, based on our findings, we want to further investigate what causes the differences between the simulations.”

A plot showing the Radial Acceleration Relation (RAR). The background is an image of the elliptical galaxy M87, showing the distance to the centre of the galaxy. The plot shows how the measurements range from high gravitational acceleration in the centre of the galaxy, to low gravitational acceleration in the far outer regions. Credit: Chris Mihos (Case Western Reserve University) / ESO
Young and old galaxies

Thus it seems that, at least one dark matter model does appear to work. However, the alternative models of gravity also predict the measured RAR. A standoff, it seems—so how do we find out which model is correct? Margot Brouwer, who led the research team, continues: “Based on our tests, our original conclusion was that the two alternative gravity models and MICE matched the observations reasonably well. However, the most exciting part was yet to come: because we had access to over 259,000 galaxies, we could divide them into several types—relatively young, blue spiral galaxies versus relatively old, red elliptical galaxies.” Those two types of galaxies come about in very different ways: red elliptical galaxies form when different galaxies interact, for example when two blue spiral galaxies pass by each other closely, or even collide. As a result, the expectation within the particle theory of dark matter is that the ratio between regular and dark matter in the different types of galaxies can vary. Models such as Verlinde’s theory and MOND on the other hand do not make use of dark matter particles, and therefore predict a fixed ratio between the expected and measured gravity in the two types of galaxies—that is, independent of their type. Brouwer: “We discovered that the RARs for the two types of galaxies differed significantly. That would be a strong hint towards the existence of dark matter as a particle.”

However, there is a caveat: gas. Many galaxies are probably surrounded by a diffuse cloud of hot gas, which is very difficult to observe. If it were the case that there is hardly any gas around young blue spiral galaxies, but that old red elliptical galaxies live in a large cloud of gas—of roughly the same mass as the stars themselves—then that could explain the difference in the RAR between the two types. To reach a final judgement on the measured difference, one would therefore also need to measure the amounts of diffuse gas—and this is exactly what is not possible using the KiDS telescopes. Other measurements have been done for a small group of around one hundred galaxies, and these measurements indeed found more gas around elliptical galaxies, but it is still unclear how representative those measurements are for the 259,000 galaxies that were studied in the current research.

If it turns out that extra gas cannot explain the difference between the two types of galaxies, then the results of the measurements are easier to understand in terms of dark matter particles than in terms of alternative models of gravity. But even then, the matter is not settled yet. While the measured differences are hard to explain using MOND, Erik Verlinde still sees a way out for his own model. Verlinde: “My current model only applies to static, isolated, spherical galaxies, so it cannot be expected to distinguish the different types of galaxies. I view these results as a challenge and inspiration to develop an asymmetric, dynamical version of my theory, in which galaxies with a different shape and history can have a different amount of ‘apparent dark matter’.”

Therefore, even after the new measurements, the dispute between dark matter and alternative gravity theories is not settled yet. Still, the new results are a major step forward: if the measured difference in gravity between the two types of galaxies is correct, then the ultimate model, whichever one that is, will have to be precise enough to explain this difference. This means in particular that many existing models can be discarded, which considerably thins out the landscape of possible explanations. On top of that, the new research shows that systematic measurements of the hot gas around galaxies are necessary. Edwin Valentijn formulates is as follows: “As observational astronomers, we have reached the point where we are able to measure the extra gravity around galaxies more precisely than we can measure the amount of visible matter. The counterintuitive conclusion is that we must first measure the presence of ordinary matter in the form of hot gas around galaxies, before future telescopes such as Euclid can finally solve the mystery of dark matter.”


Hannah - So, after the Big Bang, there was lots of matter around and gravity plus pressure caused it to clump together to form planets and moons, and suns. But how come dark matter doesn't cluster in the same way to form dark suns or dark planets, or does it? We turn to Dr. Andrew Pontzen, Cosmologist at University College London

Andrew - There's quite a lot to unpack in this question, the first thing to say is that dark matter is this substance that we're pretty sure is out there, shaping the visible contents of the universe, but the dark matter itself is invisible. So, we can't see directly what it does. Now, the question itself is getting at the idea of gravitational collapse which is a critical part of the way that we think the universe has evolved and familiar objects within it were born. Actually, dark matter does undergo gravitational collapse, so you can take an initially large volume of dark matter and shrink it down to something smaller, just because of the gravity of the dark matter itself. But the assumptions behind the dark matter tell us that unlike in the case where you have normal gas, the dark matter particles can't get rid of their energy, they you continue flying around at very high speeds. Although dark matter particles don't actually feel pressure in quite the same way that normal matter would, you can imagine they just moving so fast that they can't be concentrated into a small volume. If you tried packing them into a small box, they'd be moving so fast, they'll just fly straight out again. So, there's actually a limit to how small you can make a cloud of dark matter. You can't make dark matter collapse into a black hole for instance because you just can't get rid of the energy to make it that small. We actually think that typical clouds of dark matter are just the right size to be lurking around galaxies in what we call a dark matter halo.

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The new research reports that signs of a faint gravitational tide, known as the “external field effect” or EFE, can be observed statistically in the orbital speeds of stars in more than 150 galaxies.

The authors say the effect cannot be explained by dark matter theories, but it’s predicted by what’s known as the modified Newtonian dynamics theory, or MOND.

“What we're really saying is that there is absolutely evidence for a discrepancy,” McGaugh said. “What you see is not what you get, if all you know about is Newton and Einstein.”

Astronomers long assumed that stars orbited the centers of galaxies at speeds predicted by the theory of gravity formulated by the English physicist and mathematician Isaac Newton more than 300 years ago.

Newton based his theory that objects attract each other with a force varying according to their mass on observations of the orbits of the planets. With refinements from the theories of the German-born physicist Albert Einstein in the 20th century, it remains astonishingly accurate.

But observations of the Coma cluster of galaxies in the 1930s by Swiss astronomer Fritz Zwicky, then working at the California Institute of Technology, found it was subject to larger-than-expected gravitational forces – an effect he attributed to “dunkel (kalt) materie,” which is German for “dark (cold) material.”

When the American astronomers Vera Rubin and Kent Ford found anomalies in the orbits of stars in galaxies in the 1970s, many scientists theorized they were caused by masses of invisible “dark matter” within and around galaxies, and the idea has dominated astrophysics ever since.

By some estimates, dark matter makes up about 85 percent of all the matter in the universe. It’s said to interact with light and visible matter only through gravity, and it explains the observed anomalies in distant galaxies.

But it’s never been seen, and so far no one has fully explained what it might be, although dark matter candidates include weakly interacting massive particles, or WIMPS, primordial black holes and neutrinos.

MOND was formulated in the 1980s by an Israeli physicist, Mordehai Milgrom, to explain the observed discrepancies without dark matter.

It proposes that gravity causes a very small acceleration, not predicted by Newton and Einstein, at such low levels that it can only be seen in galaxy-size objects and it would mean the explanation of dark matter is not needed.

So far, MOND has survived several scientific tests – although many scientists say it cannot explain observations of the Bullet cluster of colliding galaxies, for example.

McGaugh admits that MOND is a minority view in astrophysics, and that most scientists favor the existence of dark matter – an idea he favored himself, until he began to change his mind about 25 years ago.

“I once would have said the same things: it’s absolutely proven that there’s dark matter, don’t worry about it," he said.

But many of the predictions of MOND have been seen in astronomical observations, and the latest research is one more piece of evidence for it, he said.

“MOND is the only theory that has succeeded in this way," McGaugh said. "It is the only theory that has routinely had all predictions come true.”

The new research raises “a very interesting issue,” said Matthias Bartelmann, a professor of theoretical astrophysics at Heidelberg University in Germany, who was not involved in the study.

“Can dark matter be explained by a different law of gravity? It would be most important for cosmology as well as particle physics if it could," he said in an email.

He has doubts, however, that the “external field effect” reported in the new research is truly a unique prediction of MOND, and that it cannot be explained by some competing theories.

And since MOND theory was formulated to account for the rotational discrepancies in galaxies, testing it on galaxies would be expected to return convincing results instead, MOND needed to be tested successfully on other objects, such as galaxy clusters, he said.

Watch the video: What is Dark Matter and Dark Energy? (June 2022).