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?
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.
In 2016, we confirmed that the spectral emissions of antihydrogen perfectly match the spectral emissions of hydrogen. When white light is emitting from a hydrogen star, it emits photons of ALMOST every color, but there are a few colors that are not emitted because the hydrogen is reabsorbing them in particular.
Since we primarily use those spectral emissions to estimate the chemical makeup of stars, we are probably completely in the dark about whether they are matter or antimatter. All we’d have to go on is the assumption that we know massive amounts of matter can exist (because of our solar system) but we’ve never confirmed that massive amounts of antimatter formed at any point after the big bang.
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.
We don’t know. However it does appear that matter out populates antimatter in the universe, leading to the theory that in the early universe matter and antimatter were created not in a 1:1 ratio, but rather 1000001:1000000. We are currently searching for evidence to confirm of dispute this. One such experiment is a satellite detecting charged particles. It has detected protons, alpha particles, and electrons for regular matter, and anti protons and positrons for antimatter. It has not detected any anti alpha particles, but if it were to even detect one it would suggest that large amounts of antimatter exist out there in the universe.
Nowhere in the observable universe have we found any evidence of structures made out of anti-matter. Mathematically, it should be there, but it’s not. You’re tangentially asking about one of the greatest mysteries in astrophysics / quantum mechanics.
We don’t know why it’s not there, we just know it’s not.