I got this really interesting setting where Humanity is reduced to a few shattered outposts in hiding and a community that has fled to take refugee into an isolationist alien empire. But I'm having trouble figuring out a believable reason as of how it ended up this way.
My initial idea was that Humanity was attacked and almost wiped out by some rampaging insectoid hive mind (like Rachni or starship trooper bugs) but that explanation just clashes with the larger galactic community setting. I want a huge amount of races living under a sort of unified galactic government. This society can't have taken in Human refugees, and need to be in peace/alliance with whatever wiped Humanity out. They would also be generally unaware about the existance of Humanity 200 years after the destruction, so it wouldn't have been a major event for them.
There are a lot of races, and Humans would've been a pretty new one when the destruction happened. That helps somewhat in making it not a big deal. And the story will be about secretly striking at whatever tried to wipe us out.
But what could have almost wiped us out in the first place? And stopped all but one isolationist empire from taking in refugees?