Summary: The ancient Mothership brought humanity to a distant galaxy.
The Mothership was ancient before our history started. It was launched from a galaxy that long ago fled far over the cosmological event horizon, leaving only a handful of archives to tell us of our origin world. It left a trail radio beacons throughout our galaxy, where it left the automated creches, seeds of humanity.
It took us centuries, but we founded a Federation across the galaxy and those in the nearby cluster. We were always too late, as the Mothership travelled at an unmatched, relativistic alacrity across the cosmos. Nevertheless, we made certain discoveries about our automated ancestor.
The galaxy it came from, the Milky Way, is far behind us in time and space. Our own telescopes have never detected it, as it vanished long over the cosmological event horizon. Each automated creche contains artificial wombs and uploaded minds of an original crew, sometimes with slight deviations to core personalities. Whether this is due to degradation of backups or an automated protocol is unknown, but it is sufficient to ensure genetic and psychological diversity at each colonization site.
We can only watch our Mother as she leaves for the next galactic cluster. She will likely arrive in a hundred million years, far after our own galaxy will be unobservable. We wave goodbye to her, for we are all children of a mother who cared for humanity for billions of years before our birth, and in her own way, billions of years after.