The Dark One, according to the guide, is imprisoned outside of the creation, outside the Pattern. He seems to be able to affect the world of the Wheel at certain ages from what we know. He is possibly affecting the world all the time although seems to get the possibility of breaking free only at certain occations.

A little beside the point:

One interesting thing is that there exist almost infinite other dimensions and mirror worlds within the Pattern. Jordan ones said that if the Bore was created in one of these worlds, then it exists in all of them and that the Dark One exists outside all of those worlds at the same time!

The Dark One is huge, I think