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?
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$\begingroup$ Well, galaxies are mostly dark matter and baryons wouldn’t have been able to condense without it, so in that sense, yes. $\endgroup$– pelaJan 3, 2021 at 23:17
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24$\begingroup$ Where did you get the idea that the universe has more antimatter than regular matter? Antimatter is comparatively rare. $\endgroup$– PM 2RingJan 4, 2021 at 13:30
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$\begingroup$ In theory, there could be antimatter galaxies out there -- they wouldn't annihilate with regular matter because of the spacing between galaxies -- they just wouldn't come in contact. However, according to Paul LaViolette's Model G Theory, antimatter is very rare and unstable because regular protons have long lives before decaying, while antiprotons have very short lives. $\endgroup$– JenniferJan 4, 2021 at 18:56
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2$\begingroup$ Mandatory XKCD: What if everything was antimatter, EXCEPT Earth? $\endgroup$– Geeky GuyJan 5, 2021 at 15:54
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3 Answers
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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.
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2$\begingroup$ Why would the "we would be quickly annihilated" argument also work for galaxies? I get it that inside the Solar System space is not empty enough and if there was antimatter it would interact... similarly for stuff inside our galaxy. But galaxies are very far from each other, so why would that argument automatically apply to them as well, if one galaxy was made entirely out of matter, and the other one entirely out of antimatter, with millions of lightyears between them. And if that's also too close, then what about galaxy clusters? $\endgroup$– vszJan 5, 2021 at 13:38
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5$\begingroup$ @vsz check the link - "The density of matter in intergalactic space is reasonably well established at about one atom per cubic meter. Assuming this is a typical density near a boundary, the gamma ray luminosity of the boundary interaction zone can be calculated. No such zones have been detected, but 30 years of research have placed bounds on how far they might be." $\endgroup$– AllureJan 5, 2021 at 19:21
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$\begingroup$ What's the mass flux from solar wind hitting Earth? I suppose one would have measurable gamma radiation if it were antimatter but I doubt it's enough to "annihilate" us, let alone "quickly". $\endgroup$ Jan 6, 2021 at 14:27
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$\begingroup$ Hmmm... a quick estimate is 3E16 W from 1 kg/s hitting Earth. Total irradiation is apparently 1.7E17 W already. So no annihilation but a significant change in the energy budget. And it should indeed be easily recognizable. $\endgroup$ Jan 6, 2021 at 14:57
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$\begingroup$ @Peter-ReinstateMonica according to the Wikipedia article on solar wind (en.wikipedia.org/wiki/Solar_wind#Acceleration), the Sun loses a million tons a second due to the wind, and radiates an Earth mass every ~150 million years. This will definitely annihilate us "quickly" (in astronomy, 150 million years is not a long time at all). $\endgroup$– AllureJan 6, 2021 at 23:24
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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.
answered Jan 3, 2021 at 12:24
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$\begingroup$ Does this limitation on condensed anti-matter only apply to the visible universe, or is there some reason to believe that the preference for matter over anti-matter is somehow fundamental? $\endgroup$ Jan 3, 2021 at 22:39
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9$\begingroup$ @SoronelHaetir As I understand things, we would expect to see radiation coming off of the boundary between matter-rich and antimatter-rich regions if any of the latter exist, and the presence of that radiation, even outside the observable universe, would have affected how the universe evolved to its current form in ways we don't see. $\endgroup$– HearthJan 3, 2021 at 22:55
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2$\begingroup$ Strictly speaking, all we know is that the area we can observe within the universe has an overabundance of normal matter relative to antimatter, not that the whole universe does, because we do not know if the observable universe is the whole universe or some small part of it. $\endgroup$ Jan 4, 2021 at 12:49
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3$\begingroup$ @Austin We know that the whole universe is larger than the observable universe, but we don't know by how much. It's (probably) at least 150× bigger, and there are good reasons to believe it's actually infinite, although (of course) we'll never be able to verify that by direct measurement. Please see astronomy.stackexchange.com/a/31795/16685 for further details. $\endgroup$– PM 2RingJan 4, 2021 at 13:28
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$\begingroup$ @AustinHemmelgarn would a matter - antimatter segregation at some larger than obs Un be theorical possible without affecting the cosmological principle? I am tempted to say yes, at least from the point at which segregation is in place. $\endgroup$ Jan 5, 2021 at 9:53
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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.
