system
Given the following post, respond as if you're commenting on Reddit:
human
If Matter and Antimatter behave very, very similar, can we be sure that distant galaxies aren't made of antimatter?

To my recollection, recent tests have shown that Anti-Hydrogen reflects light very similarly to normal hydrogen.

Can we still tell them apart over astronomical distances?
gpt
The issue is that there really isn't any "empty space".  Even in the space between galaxies you have on average 1 atom per cubic meter.  That's not much, but it's enough that if different galaxies were made of different types of matter (anti- versus regular) there would necessarily be some bright, easy to observe, "cross-over" region where the two types are meeting and annihilating each other to produce photons.  And we don't see that.

Furthermore, as we look back in time towards the big bang, every point in space was closer to every other point in space, that means that the particle densities, even between "clumps" of stuff, necessarily increases.