Since the 1960s we know that most galaxies contain a black hole in their centre. These black holes have masses in the order of millions of times the mass of the Sun, and they are called supermassive black holes.
The first black holes of this type were discovered thanks to quasars. These objects are nuclei of very distant galaxies that emit so much light, that the only way to explain such an energy source is the existence of black holes that absorb huge amounts of matter.
From that moment, studies of stellar orbits have shown that even in relatively calm galaxies such as our Milky Way, where there isn’t much activity, reside black holes with millions of times the mass of the Sun. For example, the black hole from our galactic centre, about 26,000 light-years away, is called Sagittarius A*, and its mass is equivalent to 4 million times the one of the Sun.
However, the origin of all these supermassive black holes is still unclear. One of the possibilities is that they were created in the gravitational collapse of enormous clouds of gas, from which galaxies were also formed.
It is also possible that they come from the merge of many black holes of lower sizes. Another explanation is that simply one black hole grew over billions of years, absorbing stars and other celestial bodies that orbit around it.