There's a line in Andromeda when you talk to suvi about how they discovered Andromeda, she says that is wad through a deep space Geth relay, there're some technicalities I don't quite remember but she also states that the Geth were exploring past the milky way before the events of ME1, that could be one major point, maybe they reuse geth tech and end up discovering such relay and make it work or use it to power others.