Wikipedia's article on antimatter says this:

There is considerable speculation as to why the observable universe is composed almost entirely of ordinary matter, as opposed to a more even mixture of matter and antimatter.

And on antihydrogen:

For example, excited antihydrogen atoms are expected to glow the same color as regular hydrogen. Antihydrogen atoms should be attracted to other matter or antimatter gravitationally with a force of the same magnitude that ordinary hydrogen atoms experience. This would not be true if antimatter has negative gravitational mass, which is considered highly unlikely, though not yet empirically disproven (see gravitational interaction of antimatter).

If antimatter had negative mass and generally repelled matter in large quantities that had little other forces acting them, we could have vast "pockets" of antimatter and matter peacefully coexisting, repelling one another. In fact, if antimatter looks the same "color" as matter and all we have to look at in deep space is light, for the most part, then we could have "anti-galaxies" that look the same as ordinary galaxies. It seems this idea has been considered, as Wikipedia notes:

Antimatter may exist in relatively large amounts in far-away galaxies due to cosmic inflation in the primordial time of the universe. Antimatter galaxies, if they exist, are expected to have the same chemistry and absorption and emission spectra as normal-matter galaxies, and their astronomical objects would be observationally identical, making them difficult to distinguish. NASA is trying to determine if such galaxies exist by looking for X-ray and gamma-ray signatures of annihilation events in colliding superclusters.

How would we know, in this case, that some of the galaxies we see aren't "anti-galaxies?"

Based on what I know, the thing that doesn't add up with anti-gravity is this:

Light is affected by gravity. If anti-matter's gravity is negative, then either (1) light only cares about the magnitude of gravity and is attracted to anti-matter or (2) light is repulsed by antimatter.

  1. This would be weird that light "acts" like "matter" in that it's attracted to matter and but also like "antimatter" in that it's attracted to "antimatter". A is attracted to B. B is attracted to C. A is not attracted to C. That's weird.

  2. This sounds like it could be related to dark matter.

See also arguments on Wikipedia's article on regions of the universe where antimatter dominates.

Another question addresses the issue of antimatter galaxies, but it does not consider the possibility of antimatter having negative mass.

There are (2) positively scoring answers on that question (which assumes the following: a star composed of antimatter hydrogen would fuse to anti-helium in an analogous way to our own Sun, and it would emit light and radiation at the same wavelengths as any regular matter star and would cause the same gravitational forces):

First positively-scoring answer:

"...But if there are regions of matter and antimatter in the universe, we would expect to see HUGE amounts of radiation from annihilation at the edges of these regions..."

This argument assumes that antimatter and matter regions would be close enough or connected such that annihilation would be visible. If we assume that antimatter and matter regions are gravitationally attracted to one another, like the question assumes, then this answer makes sense. My question assumes the opposite - that antimatter gravitationally repulses matter and vice-versa. Therefore the thin inter-galatic medium strands connecting gravitationally bound filaments might not exist between antimatter and matter (anymore - they've been annihilated, causing an actual void, or at least the visible appearance of one).

Here is what the second positive-scoring answer has to say:

....In case of a star made out of anti-matter you would have a burst of electron anti-neutrino emissions, and this leads to a different detection signal in detectors on Earth....

This answer admits that "the telltale sign would come from supernova neutrinos, although at present we can only detect such neutrinos from nearby galaxies". The absence of anti-neutrino emissions (today's measurements) would only imply that "nearby galaxies" are not anti-matter. What about galaxies that we can see that aren't nearby? This answer doesn't address those.

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    $\begingroup$ Possible duplicate of How would we tell antimatter galaxies apart? $\endgroup$ – John Rennie Apr 14 '16 at 6:10
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    $\begingroup$ I also made some significant edits to the question to improve clarity. @WordsLikeJared, please check and make sure you still find the question acceptable. In fact, I think some more drastic edits would help make the question even more clear, but I didn't want to go too far without your approval. If you want, I can make an additional edit to cut out the parts I think aren't necessary (e.g. most of the quotes), and I do believe it will make the question better. $\endgroup$ – David Z Apr 14 '16 at 9:53
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    $\begingroup$ @WordsLikeJared I do see the value of the quotes, and if they help your understanding, it's fine to include them. My thought was that there is an optimal balance between providing enough information to explain yourself, and not writing too much so that people lose sight of the main point while reading. Right now, I think the post is longer than optimal, and it would be easier to understand if it were shorter. (I certainly wouldn't edit it in a way that makes it a worse fit for the site!) $\endgroup$ – David Z Apr 14 '16 at 10:18
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    $\begingroup$ As the question has been closed, I will just add an answer to the title "If matter and antimatter repel, how do we know other galaxies aren't made of antimatter" in the comment. The present model of the universe is based on General Relativity. In general relativity inertial mass and gravitational mass are postulated the same. In all our particle experiments we have not seen negative inertial mass , so the principle holds. We have seen antiparticles, and generated antihydrogen and currently there are tests of whether antihydrogen falls up under gravity ( the earth is matter) $\endgroup$ – anna v Apr 14 '16 at 10:45
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    $\begingroup$ alpha.web.cern.ch/node/248 . One has to have clear in one's head that if, and it is a big IF, antiparticles have repulsive gravitational mass, the basic postulate of General Relativity, the equivalence between gravitational and inertial mass, will be falsified, so it is not a minor parenthesis. A new or a drastic modification of GR will be needed. As up to now all data and observations validate GR , the probability of finding a repulsion of matter and antimatter is very very small, imo. $\endgroup$ – anna v Apr 14 '16 at 10:49

Interstellar space is an excellent vacuum, but it's not a perfect vacuum. For example Earth is constantly bombarded with protons from the solar wind, which stream outward uninterrupted until the heliopause when matter from other stars becomes more dominant. If there were, say, an antimatter star nearby, the place where its stellar wind of antiparticles met the wind of particles from the Sun would produce a sheet of matter-antimatter annihilation radiation which would be light-years across.

The exact same argument applies to galaxies. The intergalactic medium is empty, but not completely empty, and the boundary between matter- and antimatter-dominated galaxies or galaxy groups would produce gamma-ray sources hundreds of thousands of light-years across. We observe no such structures.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – David Z Apr 14 '16 at 14:54

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