Final answer:
A singularity is a point with zero volume and infinite density at the center of a black hole, where spacetime ceases to exist and the laws of physics break down. Black holes are characterized by their singularity and an event horizon, which is the boundary from which nothing can escape. Evidence for black holes is substantial though indirect, based on their gravitational effects on surrounding space.
Step-by-step explanation:
A singularity is the point of zero volume and infinite density to which any object that becomes a black hole must collapse, according to the theory of general relativity. At the heart of a black hole lies this singularity, where the laws of physics as we understand them no longer apply, and spacetime ceases to exist. The collapsing star-core which forms the black hole is predicted to continue to compress under its immense gravitational force, ultimately forming this singularity. However, the exact nature of a singularity remains elusive, as our current understanding of physics doesn't provide the tools needed to fully describe this phenomenon.
Around the singularity, there is an event horizon, which marks the boundary of the black hole. The escape velocity at the event horizon is equal to the speed of light, making it impossible for anything that crosses this boundary to escape. While black holes may seem complex, they are defined simply by their mass, charge, and spin, making them quite straightforward in comparison to other cosmic objects.
The evidence for the existence of black holes, such as Sagittarius A* at the center of our galaxy, is compelling albeit indirect. It is inferred from the observation of massive amounts of matter confined to a very small region of space, impacts on surrounding matter, and gravitational effects that cannot be explained by the presence of any known ordinary matter.