Planar Cosmology

What most people would call the "known universe" was labeled the Prime Material Plane. It held the world, its moons, the sun, and extended out into space to the stars. How far out into space was a question of cosmology, and the answer might depend on which cosmological model was used to describe the Prime Material plane. But there were many other planes of existence, some very much like the Prime and some so alien that they almost defied description. Like other dimensions, these other planes might overlap, surround, or penetrate the Prime Material plane, or they might be connected by some mystic avenue, often requiring magic to explore. Together, all the various planes were usually referred to as the "multiverse" and they were the focus of cosmologists as they tried to map, measure, and understand the planes and their relationship to life, to death, and to each other.