Here's how it is...
Earth-that-was got used up. We moved out, terraformed and colonized hundreds of new earths; some, rich and flush with the new technologies, some, not so much. The central planets, them as formed the Alliance decided all the planets had to join under their rule. There was some disagreement on that point.
After the Unification War, many of the Independents who had fought and lost drifted to the edges of civilization, far from Alliance control. Out here, people struggle to get by with the most basic of technologies. A ship will bring you work; a gun will help you keep it.
A captain's goal is simple: find crew, find a job... keep flying.
A supply ship approaching a terraforming station.
The core worlds are full of high technology and huge cities. By contrast the recently terraformed outer worlds are generally low-tech and look more like a western movie that a sci-fi one. Food is scarce out there, and the colonisation process seemes to be one of survival of the fittest, with manpower often being chaper than food.
Many folks come from the core worlds trying to look for a new life out there. Many starve or have there hopes dashed in some other way. Many must find jobs, working for corporations and small businesses that thrive on the cheap labor. Some live in conditions of virtual slavery.
Diagram of the Verse.
Alliance patrol ships.
Some say that Reavers are monsters from another world, aliens. Some say that they are normal men who looked out into the vastness of space and lost their minds at the throught of the great nothingness beyond. Who is to say which is the more frightening? Of course, some people have seen the film.