Spheres of the Multiverse
Also called “the spaces between spheres” this is the mystic substance that binds the Spheres of Existence and many believe that the doorway to the Far Spheres, the world of the Uthras, is hidden within this “Space”. The Far Spheres are not typically included in most diagrams precisely because you cannot get to them by methods currently used by Spelljammers throughout the universe. The Firmament is actually tied into the Transitive Realms of Existence, the Plane of Shadow, the Plane of Ether and the Astral Plane. Scholars maintain that is is the source of these schismatic Planes but others say that after the splitting apart of the Transitive Realms of Existence, the Firmament is what remaining of the ethereal nothingness between lesser galaxies and the Great Spheres. This remaining transitory substance is fluid and ever changing, expanding from some unknown center of the Spheres of Existence.