Question

How can black holes be detected if they are invisible?

Answer 1

X-RAYS

Explanation:

Some black holes have a companion star. When the gases from the companion star are pulled into a black hole, the gases are heated. Before the gases are sucked into the black hole and lost forever, they may give off some X-rays. So scientists can detect black holes by the X-rays given off when matter falls into the black holes.

http://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-a-black-hole-58.html