Theoretical physics are, quite simply, physics that remain theoretical. This covers any areas of physics in which we can not actually confirm our hypotheses. This happens for various reasons; for instance, while the notion of their existence is fairly commonly accepted, black holes remain theoretical because we cannot actually confirm their existance, a) because they are located at a mind-buggering distance from here, and b) a black hole sucks everything within its event horizon, into it, which makes detecting evidence of its existence very difficult (to complicate matters, areas in which we suspect black holes may exist have been observed to emit radiation, although a possible explanation of this has been proposed (Hawking radiation.) Also, many subatomic particles remain theoretical because we have not yet generated energy levels that are high enough to produce them. The popular end of theoretical physics are the highly unlikely but theoretical hypotheses that are the bread and butter of many science fiction writers; time travel, wormholes, ansible technology, etc.