The terminology here is interesting, because astronomers have noticed a "cold spot" in our universe where they theorize another universe, on this same plane of existence, may have run into our own. It really comes down to semantics, but there may be both multiple universes traveling through our plane of existence, and parallel universes that contain different possibilities.

The existence of multiple universes is the most likely. The most accurate view of the universe is one where we assume we are not extraordinary. If our universe exists then the most likely scenario is that other universes exist. Whether they are in any way similar to our own is another question.