Black holes are under debate from a long time, but they are still difficult to understand. We have found many stellar mass black holes, but the important discussion started in 2008 when astronomers confirmed the discovery of super massive black hole. Before 2008 super massive black hole was just a theoretical concept. In our Milky Way Galaxy a super massive black hole lies in a region known as Sagittarius A*. The radius of this super massive black hole is about less than 6.25 light hours. It is a super massive black hole because a huge amount of matter is contracted in it. Its total mass is compacted into a small sphere and its mass is about 3.7 million solar masses. It is considered that super massive black holes increase their mass by capturing nearby stars and gas. The confirmation of super massive black hole in our galaxy was a result of 16 years of continuous observation. Look up in the sky at night towards the Sagittarius constellation and you are actually looking at a center of Milky Way Galaxy.
Super massive black holes are mostly present in the center of large galaxies like the Milky Way. Super massive black holes are still considered to be mysterious objects. You must have heard about quasars, they are present in the center of the galaxies and now scientists believe that Quasars are powered by super massive black holes. One reason behind the non-observable nature of super massive black hole is, they are dark and not even light can pass through it. A star around super massive black hole does not have any defined orbit, but they move like a bee buzzing in a hive. Further away from super massive black hole of our galaxy there are six stars with defined orbit. Scientists believe that stellar mass black holes actually populated the universe, but super massive black hole is dominating it.
You can imagine the existence of a super massive black hole as an invisible ninja that eats stars and planets. Formation of a super massive black hole is a still open discussion. Astrophysicists agree that there is a super massive black hole that force stellar mass black hole to merge with it. There are many hypothetical mechanisms of formation of super massive black holes. One of them is in the early age of universe black holes formed and they merged in the form of groups resulting into super massive black holes. All the process explained here or anywhere else are theoretical. A few suggestions about quasars are that they were more active in younger universe and caused the formation of super massive black holes in the early age of the universe. Whatever the process of formation of super massive black holes is, one thing is quite sure that they are not just theoretically possible but they also exist in the real world.