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

Scientists say that at the time of the big bang, in theory, equal amount of matter and antimatter should've been created, but now we have the universe of only matter. But how do we know that exactly? If antimatter has the same properties and behaviors similarly, how do we know that some Galaxy far away isn't made out of antimatter?
gpt
When matter and antimatter interact it produces gamma rays, very high energy radiation. If there were individual antimatter galaxies we would see a halo of gamma rays coming from them as dust from outside collides with them. If the universe were divided into a matter and antimatter at a larger scale we would see similar halos are larger structures.