Galaxies are large collections of stars, comprised of millions and often billions of stars. Astronomers think there are billions of galaxies in the universe. What keeps the stars in a galaxy together? Gravity, or more specifically, gravitational orbits.
A black hole in its simplest form is a collapsed, dead star. Typically a huge star, many times the size of our sun and at the end of its life will explode as a supernova. If there is enough mass left after the explosion, the star’s remnant mass collapses quickly and catastrophically in on itself to something so extremely dense that not even light can escape its grasp. Black holes can and do grow in size all the time. They suck in and crush whatever matter, gas, stars, or other black holes that fall into their intense gravitational fields.
Astronomers have been reaching the conclusion in the past few years, based on their research, that our Milky Way galaxy and some other galaxies have at their centers black holes with masses equal to millions of suns. It is not only common for large black holes to be at the center of some galaxies, as astronomers are discovering, but in my opinion this is the only way that galaxies can exist in the universe. If the stars in a galaxy do indeed orbit or rotate around a center point, as the planets in our solar system rotate around their central sun, the galactic stars must rotate around a galactic center with enough mass, and hence gravity, to hold them in their orbits.
There is no one star or thousand stars massive enough to keep millions or billions of stars in orbit. One could argue that the million stars close to the galactic center of a galaxy could perform as a gravitational point of attraction for the rest of the galaxy, and they may indeed add to the center’s gravitational influence, but so many stars would themselves need a gravitational focus or they would have no orbits and would continuously fall into each other aggregating into black holes after reaching critical mass anyway.
Therefore it is my contention that every galaxy must have a huge black hole at its center acting as the central orbital focus for all the stars in the galaxy. Not just some galaxies -- each and every galaxy that exists. And the size of the galactic black hole (or holes) must be directly proportional to the size of the galaxy and the material in orbit about it.