system
Given the following post, respond as if you're commenting on Reddit:
human
If anti-matter interacts with light the same as matter, how do we know that half the galaxies we see aren't made of anti-matter?
gpt
When matter and antimatter meet, they annihilate each other and the mass is converted into energy--specifically, into gamma-rays. If a distant galaxy were made of antimatter, it would constantly be producing gamma-rays as it encountered the matter in the intergalactic gas clouds that exist throughout galaxy clusters.


We do not see any steady stream of gamma-rays coming from any source in the sky. Therefore, astronomers conclude that there are not occasional 'rogue' galaxies made of antimatter. If there is any large amount of antimatter in the universe, it must encompass at least an entire galaxy cluster, and probably a supercluster. Once might postulate the existence of such antimatter superclusters, but then one would be faced with the problem of coming up with a mechanism that, shortly after the big bang, would have separated these now-gigantic clumps of antimatter from the neighboring clumps of mater. No such mechanism has yet been envisioned.