Great Hyperspace War Campaign
Great Second Schism
Thousands of years following the formation of the Jedi, a war exploded between users of the Dark and Light side of the Force. It was violent with the war seeing many Jedi killed. Eventually the Jedi of the Light side prevailed, gathered the Dark Jedi and banished them to the unknown regions. They have never been heard from since and many historians, Jedi included, presumed them dead or lost to the galaxy.
Jedi Master Ooroo has speculated that the Sith Empire may very well be descendants of these long banished Jedi.