How can a black hole be detected even though its gravitational field is too strong for light to escape?

Scientists detect their effect on the matter nearby, say if a normal star passes near a black hole, the black hole will pull it towards itself tearing it apart, as the attracted matter accelerates it emits x-rays that radiate in space till it reaches the Earth.

We can infer the presence of black holes and study them by detecting their effect on matter nearby.