Soul Fragments

During a Soul's trip through the Spiral of Divinity, it is subjected to material existence.
Whether it likes it or not, it ends up picking up memories and emotions, analyses and dreams, hopes and thoughts, pains and fears. Once its physical existence comes to an end and it's ready to ride the spiral again, it sheds almost all of it in the form of soul fragments.
These Fragments, essentially representing our collective unconsciousness, tend to bond together forming clusters, nebulae, matter. In most cases, they last a little, then dissipate.
Certain concepts though may be strong enough to take permanent shape. A prime example of this are works of fiction which have spoken to numerous Souls across the ages, inescapably creating fully-fledged worlds somewhere on the Spiral's length.
These Pocket Universes can be visited by souls as they ride down the Spiral.