To identify an object as a black hole, you have to first measure its mass. Reinhard Genzel and Andrea Ghez observed the movement of stars around a compact object at the center of the Milky Way. High-precision observations were conducted at the La Silla Observatory and Keck Observatory. Knowing the orbits of these stars and their period of revolution around the central object, we can calculate the mass of this object. It turned out that these stars were orbiting a massive object a million times larger than our Sun. Given that this object is invisible, it can only be a supermassive black hole. Other galaxies also have them.