Supermassive black holes exist at the centers of galaxies.

Evidence

For example, observations with the Hubble telescope of the galaxy known as M87 reveal that it contains a disk of gas 130 light-years across rotating about a central object two thousand million times the mass of the sun. — Hawking, Stephen, A Brief History of Time, 173

Categorization

• corroborated: 2 independent passages support this claim — Hawking, Stephen, A Brief History of Time, 173