Sounds bizarre. That should only happen if you ran out of planets.
That's what I thought, but that definitely didn't happen.
I was the sole inhabitant of a solar system with a culture monopoly.
If you have a save game made not much time before that happened, try loading it and see if the unexplained defeat is reproducible. If it is repro, consider uploading the save game with a bug report.
I'll do that!
Thanks for the help, guys.