system
Given the following post, respond as if you're commenting on Reddit:
human
About the matter and antimatter imbalance. Is it possible that anti matter galaxies exist beyond the observable universe?
gpt
Yes. 

This is one of the proposed explanations for the observed absence of antimatter, or the 'matter-antimatter asymmetry problem.' 

How would we observe that this is the case? Well, two ways. First, if there are large region of the universe filled with antimatter then they would be pretty much indistinguishable from the parts of the universe filled with matter. We'd expect antiprotons and antielectrons to form the same kinds of atoms we see, so we'd probably expect gravity to pull them together to make some kind of antimatter stars, which clump together in antimatter galaxies.

But if this is the case, then that means there will be boundaries between matter dominated regions and antimatter dominated regions. And we all know what happens when matter and antimatter meet...

These boundaries would be very hot to annihilation of particles in the intergalactic medium, which would probably create a sort of hot layer separating the matter and antimatter regions, a bit like how water droplets don't actually touch a hot pan because of the vapor layer that forms underneath them.

These may have structural consequences for the distribution of galaxies, and they would be resolved by an excess of high energy photons. We haven't seen it yet, but we're still looking for it. At this point it seems unlikely that they might exist, but failing to detect something for a long time isn't the same as ruling out its existence entirely.

Alternatively, experiments on earth may provide some insight. If antimatter interacts with gravity differently than matter, then we might not expect structurally similar stars and galaxies. It might, statistically speaking, lead to different large scale structure in parts of the universe that are antimatter dominated. To the best of our knowledge antimatter should 'fall' (ie interact with gravity) exactly the same as matter, but because antimatter is so tricky to produce and manipulate in experiments we don't fully know yet. Any observation of antimatter interacting with gravity differently than matter would not pass go, would not collect $200, but would go straight to Nobel Prize.