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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At one time it was considered a possibility that the universe has differentiated regions, composed solely of matter or solely of antimatter. This was one of the possible answers to the question of [baryon asymmetry](https://en.wikipedia.org/wiki/Baryon_asymmetry). However, if this were the case, there would be regions in deep space where the “predominately matter” and “predominately antimatter” regions bump up against each other. Such areas where they collide would result in both forms of matter annihilating and generating gamma rays, which could then be detected. We have yet to find some gamma ray emissions, so the theory is not considered likely.
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