The Expansion Region got the name because of how rapidly it filled with new colonies, when hyperdrives were first developed the Inner Rim was the effective edge of the galaxy due to hyperdrive range limitations. But as technology advanced, the Expansion Region was flooded with colonists looking to make a new life for themselves.
Many megacorporations made the Expansion Region their home, as a vast majority of the worlds in the region are very rich in some of the more useful materials in the galaxy. The inhabitants of the region quickly became oppressed by the megacorps as they stripped entire systems of resources, leaving the planets polluted husks of the lush worlds they once were.
Some worlds in the Expansion Region are still oppressed by megacorporations, but others were liberated by the Expansion Region Liberation Alliance. The Expansion Region is now mostly a war zone as the ERLA attempts to tear control of the worlds away from the megacorporations.