system
Given the following post, respond as if you're commenting on Reddit:
human
Is it possible that, in our universe, there are entire galaxies made out of anti-matter?

Most, if not all, universe documentaries that I have seen consistently say that we are the remnants of this great battle between matter and anti-matter. But, isn't it possible that there is still quite a bit left, and it could have formed into an entire galaxy?
gpt
The space between the stars in a galaxy is filled with a very thin gas - the "interstellar medium". Even between galaxies there is the "intergalactic medium" too. This means stars and galaxies aren't completely isolated in space - there's gas everywhere. This means you'll have matter and antimatter annihilating each other in the "border regions" between antimatter and matter galaxies. This would produce a constant stream of ~~\~2~~~1 GeV gamma rays in these border regions, which we'd be able to detect with our gamma ray telescopes. However, we don't see this.