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Galaxies are thought to be composed of matter. But what if they are composed of anti-matter? Can it be shown that they are composed of normal matter? What kind of observation could distinguish between the two? Would observations made on galaxies be different if they are composed of anti-matter. Is it even possible that anti-matter galaxies exist?

So, there have to be dividing surfaces between regions containing (eventually) anti-matter only and regions containing only matter. On these surfaces (thin volumes), a reaction between both types of matter can occur. Matter and anti-matter discombobulate, with the result that gamma rays will flow into the surrounding space. But are there enough rays to be visible on Earth? Can there be other observations that hint at the segregated states of matter (if real)? Can we ever be sure that aren't separate regions, thereby confirming the theoretical assumption that there can't be separate regions?

What is needed to detect? For sure, the detection has to be mediated by photons, or gravity. So, will there be a difference in the photons produced by a normal galaxy and an anti-one? Is it possible maybe that anti-particles interact differently with the Higgs field, giving a different mass galaxy? Which should be observable.

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    $\begingroup$ Of interest? scientificamerican.com/article/how-do-we-know-that-dista $\endgroup$
    – Alchimista
    Jun 8, 2021 at 9:10
  • $\begingroup$ @Alchimista For sure! It says that there could be no mechanism to separate the two kinds of matter. And there are no gamma flashes seen in the sky. It doesn't address the question though if the observations would be different if galaxies consisted of anti-matter. I don't see how they could be different. I can imagine that both forms have evolved separately (if not destroyed in the early universe). ;) $\endgroup$
    – Deschele Schilder
    Jun 8, 2021 at 9:25
  • $\begingroup$ physics.stackexchange.com/questions/1165/… $\endgroup$
    – Alchimista
    Jun 8, 2021 at 9:54
  • $\begingroup$ Related: physics.stackexchange.com/q/590069/123208 $\endgroup$
    – PM 2Ring
    Jun 8, 2021 at 10:52
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    $\begingroup$ I just came up with an idea - the matter/anti-matter are pretty much symmetric in electromagnetic phenomena (as far as we know), but we do know that weak interactions are not exactly the same. On a large scale, it may impose a different stellar evolution pattern - antimatter main sequence being somewhat displaced, for example. $\endgroup$
    – fraxinus
    Jun 8, 2021 at 21:45

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When you say anti-matter, I assume you mean the opposite of what our Galaxy is made of. Obviously you can label what we are made of as matter or anti-matter as you wish, the point is that the universe as we see it seems to be made (mostly) of one of those possibilities.

In principle, distant galaxies could be made of anti-matter - all the physics would be the same and there are no obvious observational signatures intrinsic to the anti-matter - but somewhere in the universe there would have to be some sort of interface where things changed from being matter dominated to anti-matter dominated. The space between galaxies isn't empty and we don't know of any way that you could partition space to keep the matter from interacting with the anti-matter at this interface.

When matter and anti-matter interact they will annihilate producing lots of high energy photons at very distinctive energies. For example, when an electron and a positron annihilate they produce two photons with energies of 511 keV. If there really is an interface between matter and anti-matter somewhere in the universe then we ought to be receiving and observing a lot of this radiation (appropriately redshifted) from this interface region.

To what extent this can be ruled out observationally, I am unsure - some authors say that existing observations of the diffuse gamma-ray flux are sufficient to rule out big regions of anti-matter within the observable universe (e.g. Canetti et al. 2012, the source cited by Wikipedia article on baryon asymmetry). On the other hand, other workers say there is still some wriggle-room by making the interface smoother (e.g. Baur et al. 2016), but these models may conflict with observations of the cosmic microwave background. But it would certainly be big news if there were large amounts of unexplained annihilation radiation coming from anywhere in space.

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    $\begingroup$ @Barbierium Yes, and we've so far seen no evidence of such high energy photons to indicate such a boundary. $\endgroup$
    – Shufflepants
    Jun 8, 2021 at 17:50
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    $\begingroup$ There is a seemingly legitimate study whose authors contend that there are 2.5 anti-stars for every one million stars in the milky way. The difficulties of this arrangement seem insurmountable to me, but maybe someone better qualified could assess it. sci-news.com/astronomy/antimatter-stars-09617.html $\endgroup$
    – JohnHunt
    Jun 8, 2021 at 17:58
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    $\begingroup$ @JohnHunt If there were really fully anti-matter stars lurking out in the universe, while collisions between stars are rare, they do happen, and if just 1 anti-matter star collided with a regular star, the energy released would be ~1000 times that of a type 1a supernova. And we can detect some of those out to 10 billion light years away with hubble. We'd see it if there was an anti-star collision almost anywhere in the visible universe, which makes the claim unlikely. $\endgroup$
    – Shufflepants
    Jun 8, 2021 at 20:14
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    $\begingroup$ @Shufflepants I tend to agree, although collisions of non-related stars have a very low probability, see astronomy.stackexchange.com/q/41000/16685 However, the paper mentioned in John's link isn't some flakey thing, it was published in PhyRevD, and the detection of anti-helium by the AMS-02 experiment is definitely pretty weird. $\endgroup$
    – PM 2Ring
    Jun 8, 2021 at 20:46
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    $\begingroup$ To what extent this can be ruled out observationally The answer is "everywhere". en.wikipedia.org/wiki/…. To quote, "it is now deemed unlikely that any region within the observable universe is dominated by antimatter". $\endgroup$
    – Allure
    Jun 9, 2021 at 4:56
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A recent article, Constraints on the antistar fraction in the Solar System neighborhood from the 10-year Fermi Large Area Telescope gamma-ray source catalog, suggests that there might well be stars made of antimatter in our own galaxy. (arXiv preprint). The claim made in this article is that the very signatures ProfRob discussed in his answer have been observed from 14 different sources in the Milky Way.

This is an extremely bold claim, and will thus need to be vetted and confirmed. The journal in which this was published, Physical Review D, is a highly respected journal, and that says something. That said, bad articles have been published in the very best journals.

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You basically answered your question with your sub-questions. Detection would be through the process of annihilation - either for individual atoms, or for a star hitting an anti-star with an unfathomably high amount of energy. We would definitely observe the latter, if it ocured in our times on a scale of planets or stars.

That said, the important phase of the universe in this respect is the very early time after the big-bang. To quote Wikipedia's article on the Big Bang, where you can also find original sources if you are so inclined:

Temperatures were so high that the random motions of particles were at relativistic speeds, and particle–antiparticle pairs of all kinds were being continuously created and destroyed in collisions. At some point, [...] leading to a very small excess of quarks and leptons over antiquarks and antileptons—of the order of one part in 30 million. This resulted in the predominance of matter over antimatter in the present universe.

As mentioned there, anti-matter is quite different from things like dark matter or dark energy in that we definitely know that it (anti-matter) does exist. We regularly observe anti-matter being created and destroyed at the scale of individual particles, both in nature and in laboratories. The only thing we cannot yet do is keep anti-matter around in any meaningful larger scale.

At the beginning, in the ur-soup of the universe right after the big bang, there was no matter, as far as we can tell - only energy. Then things cooled down (for a very eccentric definition of "cool") and the first structures we would call "matter" were created. Those whizzed around at near light speed and annihilated each other all the time - converting back to energy. Eventually, through pure quantum-mechanical chance, one type of matter won, and eventually the whole mess cooled down so much that it became stable enough to form the universe we know today.

So there you have it: the simple fact that matter and anti-matter react absolutely violently, and that we had an energy-based ur-soup at the beginning (with no clear demarcations as far as we know) means that it is highly likely that no anti-galaxies exist from a theoretical point of view; and we have never observed anything at all which would invite speculation otherwise.

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