parallel universe

noun

1. Physics. any of a hypothetical collection of undetectable universes that are like our known universe but have branched off from our universe due to a quantum-level event.See also multiverse.
2. (in cosmology) a hypothetical universe that coexists with our known universe but may operate under fundamentally different laws of physics.See also multiverse.
3. Also called alternative universe. (in science fiction, fantasy, etc.) a separate universe or world that coexists with our known universe but is very different from it.
4. a realm of existence and experience that is fundamentally different from the one that most humans share; a separate reality: I don’t understand him—I think he lives in a parallel universe.