What Is a Black Hole?

Question: What is a Black Hole?
What is a black hole? When do black holes form? Can scientists see a black hole? What is the "event horizon" of a black hole?
Answer: A black hole is a theoretical entity predicted by the equations of general relativity. A black hole is formed when a star of sufficient mass undergoes gravitational collapse, with most or all of its mass compressed into a sufficiently small area of space, causing infinite spacetime curvature at that point (a "singularity"). Such a massive spacetime curvature allows nothing, not even light, to escape from the "event horizon," or border.
Black holes have never been directly observed, though predictions of their effects have matched observations. There exist a handful of alternate theories, such as Magnetospheric Eternally Collapsing Objects (MECOs), to explain these observations, most of which avoid the spacetime singularity at the center of the black hole, but the vast majority of physicists believe that the black hole explanation is the most likely physical representation of what is taking place.
Black Holes Before Relativity
In the 1700s, there were some who proposed that a supermassive object might draw light into it. Newtonian optics was a corpuscular theory of light, treating light as particles.
John Michell published a paper in 1784 predicting that an object with a radius 500 times that of the sun (but the same density) would have an escape velocity of the speed of light at its surface, and thus be invisible. Interest in the theory died in the 1900s, however, as the wave theory of light took prominence.
When rarely referenced in modern physics, these theoretical entities are referred to as "dark stars" to distinguish them from true black holes.
Black Holes from Relativity
Within months of Einstein's publication of general relativity in 1916, the physicist Karl Schwartzchild produced a solution to Einstein's equation for a spherical mass (called the Schwartzchild metric) ... with...