Final answer:
Black holes are intense gravitational singularities from which nothing can escape, not even light. They are the remnants of supernova collapsed massive stars and have event horizons where the escape velocity equals the speed of light. Black holes possess central singularities and are supported by overwhelming circumstantial evidence.
Step-by-step explanation:
Black holes are one of the most fascinating and enigmatic phenomena in physics. They are celestial entities with gravitational fields so intense that nothing, not even light, can escape once it passes their event horizon. The concept of a black hole began as early as the late 1700s with the idea of a 'dark star' whose escape velocity exceeds the speed of light, proposed by French astronomer and mathematician Simon Laplace. The current understanding of black holes, grounded in Einstein's theory of general relativity, tells us that they can form from the supernova collapse of a massive star, leading to an object with a mass greater than that of our Sun and a size possibly as small as 10 km across.
Evidence for the existence of black holes, while circumstantial, is copious and compelling. When a star with a stellar core more massive than three times the Sun's mass exhausts its nuclear fuel, it is theorized to collapse into a black hole. At the heart of a black hole lies a singularity, a point of infinite density where all its mass is concentrated. The boundary of a black hole is known as the event horizon, which marks the point of no return, as the escape velocity at this boundary equals the speed of light.