Cataclysm
Somewhere in the late 21st century, there was a series of cataclysmic events that wiped out a large portion of the population. Among the events included a nuclear apocalypse, but scholars also note evidence of a series of mass hallucinations, mass hysteria, mega volcanoes, plagues, tsunamis, and many other things that went wrong at this time. These events are simply referred to as "the Cataclysm" for short.
Many archives and technologies were lost, leaving later archeologists and historians to piece things together. Terra's history before the Cataclysm is unclear as a result. However, this results in a lot of myth and propaganda of previous time periods merging into history, leaving humanity with an unclear view of itself. It took many generations for humanity to get back on track, though many areas of the world remained unlivable for various reasons.