I'm sure some bright scientist has come up with this before but I have a theory that what we dream with related to what we experience within parallel universes.

If we believe, as some physicists do, that the universe we live in in one of many millions throughout time and space, then it could be that we slip in and out of numerous universes during our lives.

We may be unaware that we cross over into new universes, but our sub-conscious registers the fact.

In our dreams our subconscious mind recalls these "journeys" bringing them into our mental "filing" system together with the memories of our lives in the universe we inhabit.

This theory could explain deja-vu and premonitions - events that have already happened in a parallel universe, and events we have lived through in our journeys into them, but which have not yet come to fruition in our reality.