Black holes are the strangest, mind-blowing objects found in space. The gravity in black holes is so much strong that it pulls so much that even light cannot get out. They have extreme high density. Albert Einstein first predicted black holes in 1916 with his general theory of relativity. The term "black hole" was coined in 1967 by American astronomer John Wheeler, and the first one was discovered in 1971.
Black holes are invisible because as no light can get out people can actually not see them. Special telescopes with special tools that detect x-rays, light or another form of radiations are built to locate black holes. If a star passes close to a black hole, it destroys the star and pulls it towards itself similarly if a black hole passes through a cloud of interstellar matter, for example, it will draw matter inward in a process known as accretion.
We cannot see what is inside a black hole, even if we send astronauts they won’t be able to come out or communicate us back. There are mainly three kinds of black holes. Stellar black holes occur when a huge star collapse for smaller stars, up to about three times the sun's mass. Supermassive black holes have mass equivalents to billions of suns. They lie at the center of every galaxy, scientists are unable to find the reason of formation of them but they believe they can be the byproduct of the formation of galaxy as they lie in the center. The third type black holes are intermediate black holes. Once upon a time scientist believed that black holes are only in super giant or small sizes but according to new research they also have intermediate sizes, such bodies could form when stars in a cluster collide in a chain reaction.