The origin of the Dreaming, if any, is not known. However, it seems that it was occasioned not by a dreamer, but by a dream, and that it is this dream which is the area behind the Gates.
Before the Sundering, everything was contained within the Dreaming. Even then, however, there was an area that was not of it, Ironfell. The two did not interact with each other, as if Ironfell were kept in a protective bubble. With the discovery of Ironfell, the bubble burst and it was separated off from the Dreaming, and the two mixed. Being entirely antithetical to each other (see Dreamstuff), each acted to destroy the other. Arcadia and Ironfell were now the opposite poles of the universe, so that any movement away from one meant moving closer to the other. The taint of Ironfell was almost everywhere.