Saw a really interesting documentary on m theory and parallel universes (I'll dig the name out from my hard drive at some point). Basically saying that parallel universes are occupying the same space as our universe, but on different membranes and therefore imperceptible to us. Because matter is created from each specific membrane, they also don't affect ours in any way.
Like you say, it's all completely theoretical and in this case definitely be proved because by definition they can't interact, but it fits in nicely with what I've felt from Sally and what my friends tell me they've experienced from D.