Anatomy of a Black Hole
A black hole is a pit in the fabric of spacetime. Space and time, according to Einstein’s theory of special relativity, are interchangeable parts of a thing called space-time: much as width, height, and depth are dimensions of a box, so space and time are dimensions of spacetime. Although the dimensions of space and time are relative and can change, contracting or dilating depending on your frame of reference—an effect noticeable when dealing with strong gravity or relativistic speeds—units of spacetime are absolute.
The figure above gives a cursory explanation and overview of what a black hole actually is. Not only is it a singularity, but it spins. Fast! When astronomers measure a black hole’s spin, they report the value as a fraction of the maximum allowed spin (which would be 1). The bigger member of the black hole binary in the quasar OJ 287 has a spin, labeled OJ 287 has a spin, labeled a, of 0.313, or 31.3% of its max,. What does that mean? This number is related to the angular momentum; it’s not a fraction of the speed of light. But we can turn it into a fraction of the speed of light. That is given (after some messy algebra)
We are pretty sure that black holes really do exist. Stars and gas at the centers of many galaxies orbit around invisible but incredibly massive objects, and we can tell how massive the object is based on these orbits: millions to billions of Suns’ worth of mass. Could it be that we don’t really understand gravity, and something else explains black holes? Yes, but no other ideas have worked out.
 Camille M. Carlisle, “Anatomy of a Black Hole,? Sky and Telescope (133, 2, February , 2017), pp. 16-17