So the Earth got used up.
Not that this stopped us. We left our homeworld and ventured out into the depths of space to find a new galaxy to support us, to create new Earths. And whilst it took a long time, generations living and dying aboard those giant arks that set out from Earth, we eventually found a new home.
The first worlds to be terraformed and colonised formed the Alliance; in their possession was the greatest technology, the most safe and luxurious way of life. Of course, not everyone wound up as part of the Alliance. Those on the Outer planets had it tough; life was a struggle, and every blessing was counted. But they had their freedom, their independence.
So when the Alliance decided that those out in the Rim should start abiding by their rules, there was some disagreement.
Things went downhill from there.
A war broke out; The War for Unification. On one side, the Alliance, who wanted to unite the galaxy under their rule; on the other side, the Browncoats, who wanted the Alliance to mind their own fucking business.
The Browncoats lost.
So welcome to the universe of Serenity, present day. The Outer Planets still have it tough, the Core Worlds still have it good. The Alliance 'controls' every planet now, but their reach and all-seeing eye does not extend everywhere.
Which means that every day, veterans, mercenaries and adventurers all eke out a living in the Black of space, smuggling contraband, doing dirty work for criminals, getting hired by planets to help protect them from outlaws.
Their goal is a fairly simple one, most of the time.
Find a crew. Find a job. Keep flying.
Welcome to the Black.