A celestial body with a highly dense core that emits no light is known as a black hole. Black holes are formed when massive stars exhaust their nuclear fuel and collapse under their own gravity, resulting in a singularity where density and gravitational pull become infinitely strong. Because of this immense gravity, not even light can escape from a black hole's event horizon, rendering it invisible and detectable only through its interactions with surrounding matter. Black holes can vary in size, from stellar to supermassive, and are key objects of study in astrophysics.