Galaxies contain three types of black holes
The universe is full of black holes, which exist in specific places of the galaxies, depending on their sizes. The smallest holes are the most common, whereas the biggest ones only exist at the centres of major galaxies.
Holes the size of stars
The smallest black holes emerge, when major stars explode into supernovas. The holes exist anywhere in all types of galaxies, and they typically weigh 5-30 solar masses.
Medium-sized black holes
Black holes of a thousand to two million solar masses are rare. They only exist at the centres of some small dwarf galaxies, but not in the least in all of them.
Supermassive black holes
Black holes can grow to 2 million-40 billion solar masses, such as after two galaxies collide. The holes exist at the centres of all major spiral and elliptical galaxies.