system
Given the following post, respond as if you're commenting on Reddit:
human
[DC Comics] what exactly was the ‘Crisis’ and why are the universes now separated into pre and post crisis?
gpt
There used to be infinite Earths. There aren't anymore, thanks to a dude named the Anti-Monitor who went through and wiped out all of them. (Or seemingly so, about twenty years later they revealed that there were actually *fifty-two* Earths left, not just the one, and now they've found even more than that.)

But, yeah, the Crisis was when the Anti-Monitor went through and started wiping out all of reality. The Earth that remained was dubbed New Earth and the timeline that came from it was drastically different from the more familiar Earth-1 or Earth-2, with many characters having different histories, powers, and personalities.

So that's why there's a split. A dude killed almost everyone and what remained was a totally different timeline.