In the NJO series they had a round about way of explaining it. in the enemy lines duology, luke fights this super dark jedi that was a clone of palpatines son or something, weird like that, anyway at the time corocant is in ruins and this guys who is very dumb is running around the place, at the end he fidn the ruins of the old jedi temple and you learn that the temple is sitting on something that has HUGE amounts of dark jedi energy, its doesnt explain what exactly it was but my impression was that it was like a pandoras box of dark energy, in the book decribed it as a fountian of dark energy they explained that the dark cave on dagobah was similar and the reason yoda was there was one to watch over it, and to his super like force and the caves super dark force would cancel out. so i am guessing that a super dark jedi or something died there long before the starwars event and stuff.