One of the meanings of aspect, and I believe, one of its more original ones, is the observable or exposed side, or appearance of an object from a specific angle of view. That meaning has tended to wane in common usage but survives in specialized usage, as in the aspect of a building (architecture) or the aspect of a planet (astrology).
I am looking for an antonym of aspect in that sense, as in the parts of an object which are invisible from a specific angle of view. If it exists at all, it might contain the root -spect as well, with an appropriate prefix.
However, I need to use the term figuratively so I'm also open to approximations. For example, the inside of an apparently solid object is invisible from all external angles of view and may possess a name that is analogous (but opposite) to aspect.