11ac1064_ions___Astronomy___Black_Holes__Question
[Date Answered] January 7, 2010 [Questioner (age, location)] L.Prashanth (Karnataka/India) [Answer] Black holes can be observed through its interaction with other matter, for example, by tracking the movement of a group of stars that orbit a region in space. This animation is a simulation of gravitational lensing caused by a black hole. [Question] 
If not even light can escape a black hole,it means that they are invisible. Then how do we know that there are black holes ??
Quickie Questions - Astronomy - Black Holes