as far as I know, they're primarily conceptual/mathematical
   constructs based upon the analogies we use to understand them.
   Originally analogized to planets orbiting, now analogized to
   clouds of probabilities. Do they exist physically? Well, there's
   something that seems to respond quite well to the maps/analogies
   we use when tested. I think there is some physical truth -
   something about mixing angles is coming to mind. But as far as I
   know, we're dealing with the constructs created to explain
   rather than definitive tangibles, at least in areas where we
   leave classic physics. We're working with effects of somethings
   that relate to other somethings but unless we can bounce a
   photon off of it (so to speak), any visualizations / maps we
   have are going to be limited somewhat.