We all picture a dream as a thing of hallucination, our imagination acting out so our brain is not completely dead whilst we lay pre-comatose for hours. Our rejuvenating bodies are quiet, occasionally twitching, rolling, even outbursting in kicks.
But what if our 'dreams' are more than said? What if they aren't our imagination, but the lives we live in worlds we can only touch when we aren't fully present in this one? What about when we can't remember what we dreamt? Perhaps the darkness we receive when we grasp for the once-was dream, it is dark because that world is dark. The life we lived in that universe is no longer a life we live. Dreams are such strange things, and like much in this vast world, utterly unexplainable besides the ever so boring science explaination.
I suppose no one can know why we dream with complete absolution with out being in a dream themselves.