The Black Hole is one of the most fearsome objects in nature. A black hole is an incredibly dense dead star. After such a large star burns out, it gravitationally collapses into itself. So powerful is its gravity that it collapses into an object that is smaller than an atom's nucleus. Black holes have extremely strong gravitational fields, similar to cosmic string fragments; even light cannot escape their gravitational fields - hence the name 'black hole'.

Near a black hole, this incredible gravity also causes terrible tidal stresses - more than enough to tear a starship or even a planet to shreds, and a wandering black hole is very capable of swallowing entire solars systems. It is very unlikely that any object or ship could survive falling through a black hole - it would be torn into sub-atomic particles by the gravitational stresses. There would also be very severe (and probably dangerous) time-space distortions near a black hole.

Each black hole has an associated event horizon, the boundary of which light can no longer escape. In some ways this can be regarded as the black hole's surface. In 2371 the USS Voyager became trapped by a black hole and experienced unusual time and spatial distortions, but escaped by using a dekyon beam to open a hole in the event horizon.

Black holes have been predicted by general relativity and their gravitational fields are so strong that nothing can escape it  - not even light.  A  black hole is defined to be a region of space-time where escape to the outside universe is impossible. The boundary of this region is a surface called the event horizon. This surface is not a physically tangible one, but merely a figurative concept of an imaginary boundary. Nothing can move from inside the event horizon to the outside, even briefly.

The existence of black holes in the universe is well supported by astronomical observation, particularly from studying X-ray emission from X-ray binaries and active galactic nuclei. It has also been hypothesized that black holes radiate energy due to quantum mechanical effects known as Hawking radiation.

In 2004, astronomers found 31 candidate supermassive black holes from searching obscured quasars. The lead scientist said that there are from two to five times as many supermassive black holes as previously predicted.[8]

There is now a great deal of indirect astronomical observational evidence for black holes in two mass ranges:

- stellar mass black holes with masses of a typical star (415 times the mass of our Sun), and
- supermassive black holes with masses ranging from on the order of 105 to 1010 solar masses.
- additionally, there is some evidence for intermediate-mass black holes (IMBHs), those with masses of a few hundred to a few thousand times that of the Sun. These black holes may be responsible for the emission from ultraluminous X-ray sources (ULXs).

In June 2004 astronomers found a super-massive black hole, Q0906+6930, at the centre of a distant galaxy about 12.7 billion light years away. This observation indicated rapid creation of super-massive black holes in the early universe.

In November 2004 a team of astronomers reported the discovery of the first intermediate-mass black hole in our Galaxy, orbiting three light-years from Sagittarius A*. This medium black hole of 1,300 solar masses is within a cluster of seven stars, possibly the remnant of a massive star cluster that has been stripped down by the Galactic Centre. This observation may add support to the idea that supermassive black holes grow by absorbing nearby smaller black holes and stars.

In February 2005, a blue giant star SDSS J090745.0+24507 was found to be leaving the Milky Way at twice the escape velocity (0.0022 of the speed of light), having been catapulted out of the galactic core which its path can be traced back to. The high velocity of this star supports the hypothesis of a super-massive black hole in the centre of the galaxy.

The formation of micro black holes on Earth in particle accelerators has been tentatively reported,[12] but not yet confirmed. So far there are no observed candidates for primordial black holes.

The term black hole was not used officially until 1967 (when stated in a lecture by Dr. John Wheeler). Prior to that time several different terms were used officially for these objects, among them "frozen star" and "black star". The term black star is used in the Star Trek: The Original Series episode "Tomorrow is Yesterday".

Although 21st century science would say that all black holes have singularities in them, it would also say that not all singularities are black holes. Differences in usage in Star Trek supports this understanding.