In the beginning, there was only one. A single black infinitude... so cold and dark for so very long... that even the burning light was imperceptible. But the light grew and the infinitude shuddered... and the darkness finally... screamed, as much in pain as in relief. For in that instant a multiverse was born. A multiverse of worlds vibrating and replicating... and a multiverse that should have been one, became many.

The Multiverse

It's a huge, endless metaphysical space with the infinite amount of different universes.