A black hole is a region of space in which the gravitational
field is so powerful that nothing can escape after having
fallen past the event horizon.

The name comes from the fact that even electromagnetic
radiation (e.g. light) is unable to escape, rendering the
interior invisible.

However, black holes can be detected if they interact with
matter outside the event horizon, for example by drawing
in gas from an orbiting star. 

The gas spirals inward, heating up to very high temperatures
and emitting large amounts of radiation in the process.

While the idea of an object with gravity strong enough
to prevent light from escaping was proposed in the 18th century,
black holes as presently understood are described by Einstein's theory
of general relativity,developed in 1916. 

This theory predicts that  when a large enough amount of mass is
present within a sufficiently small region of space, all paths through
space are warped inwards towards the center of the volume,
forcing all matter and radiation to fall inward.

While general relativity describes a black hole as a region of empty
space with a pointlike singularity at the center and an event horizon at
the outer edge, the description changes when the effects of quantum
mechanics are taken into account. 

Research on this subject indicates that, rather than holding captured
matter forever, black holes may slowly leak a form of thermal energy
called Hawking radiation.

However, the final, correct description of black holes, requiring a
theory of quantum gravity, is unknown.