Most physicists believe in the existence of black holes which have become the topic of interest within the scientific community during much of the twentieth century. Albert Einstein and his contemporaries denied the reality of such object; however, modern physicists continue to base their views on a growing body of observations. The term black holes was coined by the American scientist, John Wheeler in 1967, although the concept of it had dated back more than two hundred years.
According to Einstein's general theory of relativity, space and time together can be regarded as forming a four-dimensional space called space-time, curved by the matter and energy in it. Also according to the theory of relativity, nothing can travel faster than the speed of light but most compact objects imaginable are predicted by General Relativity to have such strong gravitational force that nothing can escape them- not even light. For this reason, it is called a black hole; light, as a result disappears from the visible universe.
John Mitchell, an English geologist, had written a paper in 1783 about the extraordinary strength of gravity- so strong that light could not escape it. Gravity this strong could have been generated from what he called "dark stars", objects that would have to be very massive and extremely dense to have this kind of force. Black holes are thought to be formed from massive stars that collapse from their own gravity to form an object whose density is infinite. During the lifetime of the star, nuclear fusion releases electromagnetic radiation, including photons. When all of the remaining nuclear force is used up, the star's core collapses. And if the star is sufficiently great in mass, it has the possibility of becoming a black hole; if not, it becomes either a white dwarf or a neutron star.
One potential black hole (one of the first discovered in the 1960's seriously considered by scientists) was named Cygnus X-1, a mysterious source of X-ray emissions.