Come back here often to read what they say.
Black holes have mass, which makes the presence of a black hole felt by any object near it.
In the former case the orbital motion of a normal visible star is measured and allows astronomers to determine the mass of the dark object around which the star is moving. There are about a dozen such systems binary known in which the dark star is approximately 10 times more massive than our sun. Astronomers think these massive dark stars are black holes. Another setting is present in the nuclei of many galaxies, including our own. This gas is so hot that its radiation can be detected.