I for one believe in ghosts, but not as spririts of dead people (at least not most of them )...
Not for sure where I read this anymore, but it said that when God created the Universe there were 12 (might have been 18) dimensions, but only 6 left after everything was created.

Building on that, my opinion is that in those dimensions are heaven, hell, purgatory, and anything else there could be...such as alternate universes...I think that when someone sees a ghost, all their seeing is a being from another dimension, that somehow got projected to this one.

Some ghosts seem to be able to interact with this dimensions, and my explanation for that is that their soul was sent to the wrong dimension (Earth instead of heaven/hell/purgatory), and have minimal capabilities to interact...

That's all I can remember of my therory at the moment