I know that when matter and antimatter collide, they eliminate each other. So it would make sense that none of the stars in our galaxy would be antimatter since it was all part of one big cloud at some point.
But if antimatter and matter behave more or less the same as matter, how much certainty do we have that distant galaxies aren’t composed of antimatter instead of matter?
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Empty space is really empty, but it isn’t *that* empty. If there were antimatter-dominated regions of our Universe, we’d see a bunch of gamma rays coming from the boundaries between antimatter-dominated and matter-dominated parts of the Universe. Since we don’t observe this, it does not appear that other parts of the observable Universe are made of antimatter.
It is, however, possible that antimatter-dominated regions exist *outside* of our observable Universe, such that the Universe as a whole (but not the part of it we can observe) has equal amounts of both. It’s worth noting that our own Universe only had a tiny, tiny bit more matter (about 1 part per billion) than antimatter, and that almost all of both annihilated one another at the beginning of our Universe. So it’s possible that what we see as matter in our Universe is the result of a very tiny difference in concentration in the early Universe, blown up to such a scale that we can’t see the unevenness in other parts of it.
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