The existence of supermassive black holes at the centers of galaxies is supported by a combination of observational evidence and theoretical models. While supermassive black holes cannot be seen directly because they do not emit light, there are several compelling lines of evidence that indicate their presence.
Stellar Motion: Astronomers have observed the motion of stars near the center of galaxies. By tracking the orbits of these stars, they have found that they move at extremely high speeds around a central, invisible object. The gravitational pull required to generate such speeds is best explained by a supermassive black hole.
Gas and Dust Accretion: As matter falls into a black hole, it forms an accretion disk—an orbiting disk of gas and dust that gets heated and emits various forms of radiation, including X-rays. Observations of intense X-ray emissions from the centers of galaxies are consistent with the presence of supermassive black holes.
Jet Formation: Supermassive black holes can produce powerful jets of high-energy particles that are visible in radio wavelengths. These jets are often observed extending from the centers of galaxies and are believed to result from the interaction of the black hole's gravitational pull with surrounding matter.
Galaxy Evolution Models: The formation and evolution of galaxies are better understood when supermassive black holes are incorporated into theoretical models. Simulations and models of galaxy evolution indicate that the growth of supermassive black holes at the centers of galaxies is a natural consequence of their formation and subsequent interactions with their host galaxies.
While direct observation of a supermassive black hole remains challenging, these various lines of evidence, combined with our understanding of astrophysics and general relativity, strongly support their existence at the centers of most galaxies.