Mmm, there's a ton of info out there, but I'm not sure how condensed or easy to find it is. From memory:
In the misty pasts of human history, things were not very organized. As humanity advanced, they developed, amongst other things, widespread genetic engineering. As with any great invention, this worked fantastically right up until it rose up and devoured us all.
Specifically, a certain strain of genetic modifications suddenly and simultaneously transformed their bearers into ravening flesh horrors, which then began to devour and reproduce and spread and adapt as ravening flesh horrors are wont to do. I'm actually still not positive how much of what we know is truth and how much is UWM whitewash, but they apparently crafted living flesh ships to infest new planets, and once they were on a world it was largely a matter of glassing it. Sometimes not even that worked; one world they scoured clean, and somehow the Altered remained. Another (it might have been the same one) they just gave up and crashed into the gas giant it was orbiting, I think.
The UWM, or United Worlds of Man, was organized specifically to unite the remnants of humanity against this threat. One of its problems is that it started as a brutal cooperate-or-we-die military autocracy that accomplished its goal but obviously didn't really want to disband, which tends to go to poorer and poorer places over time. Possibly as a side effect of this, publicly available information tends to be spotty or misleading; I assume we have access to better stuff now that we're officially ARM, but I'm not sure if anyone's gone after it.