The Expansion was an era of colonisation by humans when Earth was on the brink of a period of five thousand years of inhabitability. Though in the following millennia the Earth was forgotten and far from the trade routes, humanity thrived throughout its many colonies.
It was around this time that the first star-pioneers reached Andromeda. Eventually that whole galaxy became dominated by humans. Another particularly successful colony was GalSec, which in ten thousand years carved out an empire with bases throughout the Mutter's Spiral. (TV: The Sontaran Experiment) The time was marked by war between GalSec and the Wirrn. (AUDIO: Wirrn Dawn)