Perhaps this will revive the O'Neil Space Colony concept
Gerard O'Neil came up with the free-floating space colony concept in the 1970s, which would use materials from the Moon and planetoids and contain potentially thousands of people.
I can see some groups one day building their own colony far away from Earth, or even stick a rocket engine on it and turn it into a multigenerational starship. Maybe that's how the galaxy has had life spread through it, by groups of beings who weren't happy with conditions on the home world.
Hey, it's one big reason why the New World was settled.
http://lifesci3.arc.nasa.gov/SpaceSettlement/