why are black holes black?

Black holes look “black” because their gravity is so strong that even light cannot escape from inside their boundary—the event horizon. Since no light can break free, they do not emit or reflect light that could reach us, so they appear dark against the surrounding universe. What we actually observe are indirect signs: hot, glowing gas in the surrounding accretion disk, gravitational effects on nearby stars and gas, and sometimes jets shooting out from the region around the hole. These observational clues let scientists infer the presence and properties of black holes even though the holes themselves are invisible.