The axiom proposed in the previous entry, that nothing exists outside of nature, must apply even to theoretical realms, regions of space, materials, phenomena, etc., that lie somewhat outside the scope of current theory or have so far not been directly observed, like parallel universes, hidden dimensions, heretofore undetected particles, dark matter and dark energy; provided, of course, their existence can be conclusively demonstrated.

In actuality it makes little sense to apply such attributes as "parallel", "alternate", "dark", or "multiple" in relation to the various proposed extentions to the portion of the observed universe already described to the general satisfaction of scientists, as any such extension would apply only to the human understanding of the thing, and not to the thing understood.  

However exotic or strange a phenomenon we might encounter, each new discovery is necessarily contiguous or coexstensive with aspects of the universe already known and understood, even if a discovery should radically alter the theoretical basis of that understanding. Otherwise, we would have no choice but to consider that the universe (or "nature", in the sense I have been using the term) is only as extensive as our limited conception of it presently allows!