system
Given the following post, respond as if you're commenting on Reddit:
human
If the ratio of matter to antimatter during the Big Bang was the opposite (1 billion matter particles to 1 billion and 1 antimatter particles) then is there any reason that we wouldn't have a universe made out of antimatter?

Saw a video about matter and antimatter by Minute Physics and got me thinking.
gpt
As far as we know, the universe would probably look almost identical - just made of antimatter. But in all likelihood, an observer in that universe would definite matter and anti-matter oppositely. There would be some (very small) measurable differences, though. Matter and antimatter do behave slightly differently in weak interactions (and possibly in strong force interactions) through what are called CP violating processes. 

It's worth noting that while anti-matter seems to behave almost exactly like matter, with the exception of the aforementioned CP violating processes, there's a lot that we still don't now about it. In particular, we don't know for sure if anti-matter interacts via gravity the same way as matter does, and it is very difficult to determine experimentally. We generally assume that it does, because it would be very weird (and very interesting!) if it didn't, but it is still technically an open question!