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.