system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know for certain that the observable universe is almost entirely matter, and not partially antimatter?

I've heard that one of the puzzling thing about antimatter is why it wasn't favored in the creation of our universe, and that mostly matter was produced in the Big Bang.

I've also heard that antimatter can form anti-planets, anti-stars and pretty much any other object that matter can.

So then, the obvious question follows: since the universe is so very spread out, and most objects never come into contact with each other, why are we so sure that all the stars and galaxies we're looking at in a particular part of the sky are made entirely of matter and not antimatter? How could one know the difference at that distance?

**In short, how can we prove that every star and planet in the observable universe is, in fact, matter and not antimatter?**

It seems like a pretty big assumption that we're making to me.
gpt
Intergalactic space isn't exactly empty. It's filled with incredibly sparse gas and dust etc. At the boundaries between matter and antimatter galaxies (or galaxy clusters or even bigger structures) there should be enough annihilation events to produce a tell-tale gamma-ray signal, which is not observed.

In addition, galaxies collide with eachother, and there's no evidence of an antimatter galaxy ever colliding with a matter one.

Lastly, going from the Big Bang to an incredibly inhomogeneous distribution of matter and antimatter to the point where whole antimatter galaxies could form requires a rather exotic mechanism. It really raises more questions than it answers.