That reminds me of a very short lucid experience I had a long time ago when I first saw the movie Waking Life. I was in a space like you describe but instead of being white and bright it was grey and dull. My guess would be that it is like a transition from one dream to another but I really have no idea. Perhaps its just an unstable dream and with more dream control and stabilization a dream scene may have formed. I'm still learning as well so I cant say for sure, but it sure is interesting.