I've been rewatching Red Dwarf lately, and it's a great show. One of the things that I've always found intriguing is the lack of humans in the show. Where did they go? When did they disappear, and why did it happen?

The show is set 3 million years after the 22nd (?) century. Some things I've noticed in the show include:

• The remains of human civilization are everywhere. I'm not sure how large a section of the Milky Way the characters are able to explore, but everywhere they go, it seems like there are relics of human civilization. They find old derelict ships and space stations, manmade AIs, GELFs (genetically engineered lifeforms), automatons, Simulants, and other human creations. But there isn't an actual human in sight.

• There are no aliens. In fact, Earth seems to have been the only planet to have ever had life. All of the habitable planets they encounter appear to have been terraformed by humans, and there doesn't seem to be any non-sentient alien life, like a planet similar to Earth that developed its own life that evolved independently. Not only are there no extraterrestrial alien civilizations or sentient ETs, but it appears that Earth really was unique in the galaxy.

• The show is set approximately 3 million years in the future. They encounter human artifacts from a range of time periods, but it's unclear how long humans were around, or exactly when they disappeared. Lister is from the 22nd or 23rd century. If I remember correctly, Kryten is from the 25th century. It's not clear what the most recent artifacts they encounter would be, but it's clear that there were further technological developments after Lister's time, and probably after Kryten's time as well. However, it's not clear when different tech (e.g. time travel, the creation of GELFs) actually dates to, or was actually invented.

Clearly, humans once thrived. Alone in the galaxy (or at least, in whatever parts of it they were able to expand into), they spread far and wide among the stars, bringing advanced technology with them. They left terraformed planets, GELFs, and truly sentient AIs behind, but not a single human is left. What the hell happened?