Do parallel universes exist? We might live in a multiversebr br (in cosmology) a hypothetical universe that coexists with our known universe but may operate under fundamentally different laws of physics. See also multiverse. Also called alternate universe, alternative universe . (in science fiction, fantasy, etc.)br br Unfortunately, we don't know if these parallel universes exist. Or at least, we don't at the moment. Physicists are trying to find ways to test these theories, but it is very difficult. For now, they remain just theories.