The Key Points:
1. Supermassive black holes exist in the center of every galaxy, including the Milky Way.
2. Supermassive black holes play the key roles in the formation of stars and galaxies.
3. Supermassive black holes become quiet after pushing out the galaxy stars by strong winds of radiation from the hot and compressed gas falling into the black holes.
4. The mass of supermassive black holes in the centers of galaxies are in direct proportion to the velocity of the stars at the edges of galaxies.