nostr:npub1ue84a9qckwkrfunjap2rh7gzdm7wsvzpcg7ux8nxtr6qsdy4rtrs4245vs - if our galaxy had significant amounts of antimatter in it, we would know that, because occasionally this antimatter would bump into matter and annihilate, releasing gamma rays with specific energies.  Physicists have spent a lot of time looking for these gamma rays, and other such evidence, and they haven't found it, so we feel very sure that our galaxy has almost no antimatter in it.  Similar studies have ruled out the possibility that clusters of galaxies or even superclusters contain significant amounts of both matter and antimatter.    So if the Universe contains more than a tiny bit of antimatter, we would need entire superclusters made of antimatter, besides those made of matter.  And it's very hard to come up with a workable theory where, starting from the dense hot gas in the early universe, we wind up with some large regions made of matter and others made of antimatter.

What we actually believe happened is very interesting: originally there was a lot of matter and antimatter all mixed up, but almost all of it annihilated, and there was a tiny excess of matter left.  The theories for how we got this tiny excess of matter are connected to the CP violation I discussed, which makes matter slightly different from antimatter.   For more:

https://en.wikipedia.org/wiki/Baryogenesis