Even though nature tries to shield the singularities of black holes, scientists know that black holes exist. In the direction of the constellation Sagittarius, at the very center of our galaxy, sits a supermassive black hole that weighs as much as two-and-a-half million suns. Astronomers have watched stars dance around an invisible partner; the stars’ motions reveal the presence of the black hole even if the black hole is not visible.
