A black hole is an intense gravity in a region of spacetime, which is so strong that even light does not escape from it. As even light cannot escape from it, a black hole acts as an ideal black body. There is a region around the black hole, from where nothing can escape, the boundary of this region is called the event horizon.
Black holes can be formed when a star collapses at the end of their life cycle. Because of its high gravitational pull, a black hole absorbs the surrounding masses, like other stars and its size continues to grow. The boundary in spacetime, after which there is no escape is called the event horizon. Any particle passing through the event horizon has no chance of return.
General relativity explains that the path taken by particles bends towards the mass as the presence of mass deforms the spacetime. Black holes in spacetime has an impact on time. Time near the black hole appears to spend more slowly, than further away. This is known as gravitational time dilation.
At the center of a black hole, the curvature of spacetime becomes infinite, this is a gravitational singularity. Gravitational singularity is a single point for a non-rotating black hole and for rotating black hole it is a ring singularity lying in the plane of rotation. Singular region is considered to have an infinite density, because it has zero volume. It is also considered to contain all masses of the black hole solution. The existence of a black hole is inferred from its gravitational influence on surroundings. Light paths get highly bent near the event horizon of a black hole.
It is believed that a black hole lies at the center of milky way galaxy and virtually in all large galaxies. Black holes are classified as primordial stellar and supermassive, based on the range of sizes. The smallest ones are primordial black holes. These black holes are small as an atom with a very huge mass. Medium-sized black holes are stellar. The size of these black holes has a diameter of about 10 miles and a mass of about 20 times greater than the mass of the sun. A supermassive black hole at the center of Milky Way galaxy is called Sagittarius A, It has a mass of about 4 million Suns and a diameter of about the size of the Sun.