system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know that there aren't entire antimatter galaxies?

Do we know that there aren't?
gpt
you'd see the "boundary" of that galaxy everytime a particle of matter crossed it and annihilated with the anti-matter.  

Concidering were don't see rings/spheres of constant explosion, means there isn't such a galaxy in the visible universe.