At the center of most galaxies, including the Milky Way, supermassive black holes have masses from millions to billions of times that of the Sun. Their gravitational pull governs stellar motion in galactic cores.
It matters because supermassive black holes play a key role in galaxy formation and dynamics.
It also helps explain active galactic nuclei and quasars.
The Milky Way’s central black hole weighs over four million solar masses.
[ESA, esa.int]