The multiverse (or meta-universe) is the hypothetical set of possible universes, including the universe in which we live. Together, these universes comprise everything that exists: the entirety of space, time, matter, energy, and the physical laws and constants that describe them. The universe we live in may not be the only one out there. In fact, our universe could be just one of an infinite number of universes making up a "multiverse." Though the concept may stretch credulity, there's good physics behind it.