Expansion

The Expansion was an era of colonisation by humans when Earth was on the brink of a period of 5000 years of inhabitability. Though in the following millenia the Earth was forgotten saw itself located far from the trade routes, humanity thrived throughout its many colonies.

It was around this time that the first star-poineers succeeded in reaching Andromeda, and eventualy that whole galaxy became dominated by humans. Another particulary succesfull colony was GalSec, which in 10,000 years managed to carve out an empire with bases throughout the Mutter's Spiral. (DW: The Sontaran Experiment)
 * It is unknown exactly at which point in history the expansion occurred, though from The Ark in Space it can be assumed that it was somewhere after the 30th century, and was directly followed by a period of 10,000 years where Earth was out of the picture.