The Many-Worlds Interpretation (MWI) of quantum mechanics holds that there are many worlds which exist in parallel at the same space and time as our own. But this is just a theory. There is no direct evidence that suggests parallel universes are real, just as there is no evidence to suggest they are not.
When you think about it, the explanation that ghosts are people or beings in another dimension or living in an alternate reality isn’t much different than believing that spirits are simply lingering souls that have not passed over. Neither of these explanations have been proven as fact, and both have yet to be unproven. Who’s to say that the ghosts we see are not ourselves from a different timeline? If you follow Einstein’s Theory of Relativity, our past, present, and future may all co-exist. Is there an alternate dimension that is relieving our past or living our future? Or perhaps another dimension featuring us living an alternate path to the one we chose?