Read this excerpt from "Why Are They Called Black Holes?" Scientists cannot see gravity, but they can see its effects. The gravity of a black hole will swallow matter and light that drift close enough to it. Some light passing by a black hole can be far enough away that it could avoid being sucked in, but the path of the light would still be bent by gravity.

Astronomers can observe these effects. For example, when a black hole passes through a cloud of dust, it pulls the dust inside it like a big gravity-powered vacuum cleaner. When a star passes too close to a black hole, the black hole pulls it in and breaks up the star, spewing giant flashes of high energy called gamma-ray bursts. And when scientists see light bending even though there is no visible matter nearby, they have evidence that there is an invisible black hole.

What must be present for scientists to detect black holes?

a cloud of dust
a gamma ray burst
the loss of a star
the force of gravity