Here's why:

* Gravity: Black holes have such incredibly strong gravity that even light, the fastest thing in the universe, cannot escape their pull.

* Event Horizon: This is the boundary around a black hole where the escape velocity equals the speed of light. Anything that crosses this boundary, including light, cannot escape.

How we detect black holes:

Even though we can't see them directly, we can detect black holes through their effects on their surroundings:

* Gravitational lensing: The strong gravity of black holes bends light from objects behind them, making them appear distorted or even multiple images.

* Accretion disks: As material falls into a black hole, it forms a swirling disk of gas and dust that heats up and emits X-rays.

* Jets: Some black holes launch powerful jets of matter and energy out into space.

* Orbital motions: The gravitational pull of a black hole can be observed by the way stars and gas clouds move around it.

So, while we can't see the black hole itself, we can study its effects on the universe around it.