Is our universe the only universe?

Not necessarily. Just because we don’t physically observe any other universes out there doesn’t mean they are not there! Our three-dimensional universe may be only one of many 3D universes.

How can this be the case? These other universes may lie within extra dimensions that we are unaware of. Admittedly, this is a rather difficult idea to visualize since we only live in a three-dimensional world! So we need a method of illustrating other universes.