20th century astronomers have predicted dark areas in the space. The gravitational attraction of these areas is so high that anything, which goes into it, cannot come out. Even light cannot escape the gravitational pull of these areas. Hence they are not visible. These areas are called Black Holes or collapsars. A German astronomer, Karl Schwarzschild predicted the existence of Black Holes in 1907. He theoretically proved that Black Holes are the end results of all stars whose mass is much greater than that of the Sun. J. Robert Oppenheimer and Hartland S. Synder, using the General Theory of Relativity, were the first to prove theoretically the existence of Black Holes, in 1939. Let us consider a star whose mass is greater than that of the Sun. Its size remains normal due to the balance between the two forces — one being the expansion force caused by the enormously high temperature which tends to expand a star’s material, and the other being the enormous gravitational pull which tends to contract the star’s substance. At some stage in the star’s life, say after thousands of millions of years, its nuclear fuel decreases causing a fall in its core temperature. As a result, the gravitational pull overcomes the expansion force. The star gradually begins to collapse due to this phenomenon, called ‘implosion’. In this process, the atoms present in the star break into electrons, protons and neutrons. The mutual repulsion between the electrons prevents further contraction. The star, at this stage, is known as a ‘White Dwarf’. In this process, the star is reduced to one-hundredth of its original size. Thereby, the gravitational pull in the White Dwarf becomes about 10,000 times more than the original value. The mass of White Dwarf stars is always less than 1.4 times the mass of the Sun. Under certain conditions, the gravitational pull becomes strong enough to overcome the electron repulsion. The star begins to contract further and during the course of contraction, electrons and protons combine to form neutrons. The star at this stage is called ‘Neutron Star’. Its size is now reduced to five-hundredth part of the dwarf star and the gravitational attraction becomes about 100,000,000,00 times that of the original star. The light emitted from the neutron star reduces its energy and as a result its size further decreases. At a particular stage, no radiations come out of this star. It is then called a Black Hole, which is the smallest, densest object in the Universe. Scientists are still looking for evidence of the actual existence of black holes in the Universe. They have identified Cygnus X-1 as a Black Hole in 1974. In 1983, US astronomers detected another X-ray source in the large Magellanic cloud.