We cannot glimpse what lies inside the event horizon of a black hole because light or material from there can never reach us. Even if we could send an explorer
into the black hole, she could never communicate back to us.
Current theories predict that all the matter in a black hole is piled up in a single point at the center, but we do not understand how this central
singularity works. To properly understand the black hole center requires a fusion of the theory of gravity with the theory that describes the behavior of matter
on the smallest scales, called quantum mechanics. This unifying theory has already been given a name, quantum gravity, but how it works is still unknown. This
is one of the most important unsolved problems in physics. Studies of black holes may one day provide the key to unlock this mystery.
Einstein's theory of general relativity allows unusual characteristics for black holes. For example, the central singularity might form a bridge to another
Universe. This is similar to a so-called wormhole (a mysterious solution of Einstein's equations that has no event horizon). Bridges and wormholes might allow
travel to other Universes or even time travel. But without observational and experimental data, this is mostly speculation. We do not know whether bridges or
wormholes exist in the Universe, or could even have formed in principle. By contrast, black holes have been observed to exist and we understand how they form.