Keepers: Infinity Spirits
Also known as the Pysical Realm, the multiverse is the combined creation of the three Infinity Spirits, although it was initially designed by Colour.
It exists separately to the three other realms, although all can be accessed from it using a variety of means.
It's main purpose is to aid in the growth and development of the souls that visit it. Matter can only be moved from and between universes by Hipothonlians, Starshifters and Supreme Beings.
As the multiverse has no keeper, it is tended to by all three Infinity Spirits and their souls and guardians.