What's your story for the aliens?

I don't explain them at all because I don't play them. They don't even exist in my game anymore, thanks to mods. I used to think they'd be a fun addition, but they turned out to annoy me with those balloon-fueled voices and to generally irritate me in other ways as well. So I got rid of them.

I like your "Xenons" story, though. Half-Life was a very cool game, too. :)