Final answer:
Black holes are characterized by their event horizon, a point of no return where the escape velocity equals the speed of light. They form from the supernova collapse of massive stars, with the center of a black hole containing a singularity of infinite density. Black holes are hard to observe directly but have strong indirect evidence supporting their existence.
Step-by-step explanation:
Black holes are astronomical objects defined by a feature known as the event horizon, which acts as a boundary from which nothing, not even light, can escape. Described by Einstein's theory of general relativity, the event horizon is a result of extreme warping of space by a black hole's intense gravitational pull. Essentially, the event horizon is the surface at which the escape velocity equals the speed of light, this radius being the Schwarzschild radius. The concept of escape velocity is crucial for understanding black holes. In the space around any large mass, the escape velocity is the speed an object must reach to break free from the gravitational pull of that mass. As proposed by Simon Laplace in the late 1700s, if the escape velocity exceeds the speed of light, no light can escape, thus creating what we now refer to as a black hole. At the very center of a black hole lies the singularity, a point of infinite density and zero volume where matter is crushed under immense gravitational force. However, the singularity cannot be observed directly, as it lies within the event horizon. Theories suggest that black holes can form from the supernova collapse of massive stars, and this has been supported by various indirect observational evidence despite black holes being challenging to detect due to their nature of not emitting light.