Final answer:
Evidence for supermassive black holes includes observations of vast amounts of mass packed into small volumes at galactic centers, growth of black holes in crowded regions, and findings from the Hubble Space Telescope and X-ray observations indicating the presence of black holes ranging from millions to billions of solar masses.
Step-by-step explanation:
Scientists have gathered significant evidence that supermassive black holes exist, with the core of our Galaxy believed to contain a black hole with a mass equivalent to 4.6 million Suns. A key observation is that this mass is packed into an incredibly small volume, one not larger than the orbit of Mercury, ruling out the possibility of it being anything other than a supermassive black hole. Observations have also shown that in crowded regions, black holes can grow by swallowing thousands or millions of times the mass of the Sun, leading to supermassive black holes that are billions of solar masses in size. Furthermore, tools like the Hubble Space Telescope have provided dramatic evidence of such black holes in the centers of many other galaxies, while X-ray observations indicate the existence of middle-weight black holes.