This made me think of a theory... What if the "Ansem" that possessed Riku (and Terra earlier, giving rise to "Xehanort") is a being residing in the deepest depths of the darkness, so that those diving into it (Riku, Terra, and possibly Vanitas earlier, thanks to Master Xehanort) make contact with it and eventually get possessed. This would explain why "Xehanort", even though he had lost all of his memories, was restlessly searching for the darkness. I guess that being would be a "Cloud of Darkness" of sorts - disrupting the balance between light and dark, striving for everything to return to the void...

Of course, this is all just pure speculation. It might all turn out to be false if Terra=/=Xehanort.