In reference to my earlier post on a Schellingian weird realism, Zizek actually articulates this quite well in his Pervert’s Guide to Cinema, as seen in this clip.
All that I would add is that it is not only human beings that are haunted by “alternate versions of themselves,” but that all of reality is in some sense haunted both by what has been as well as what could have been. The whole universe then possesses this quality of spectrality, and we are all haunted by the spectral universe.
[ADDENDUM:] I want to add that I am not speaking of spectres as mysterious things-in-themselves, but rather what Schelling refers to (and Iain Hamilton Grant picks up on) as the retarded (that is, impeded) productive activity of Nature. What this means is that there are parts of Nature that are incomplete or stunted and appear as possible voids or ghosts in reality. The universe is itself imperfect.