Calamity

Millennia ago, the Calamity killed nearly all life and plunged the universe into a dark age. Even now, technology has not reached the level it was at back then. No one knows for certain what the Calamity was, all records of its nature or what triggered it have been wiped from history. All that is known is it came from another plane. This caused a universal ban on any contact with other planes, a rule which has survived to this day.