Black holes are mostly places where stars die in the universe. If you want to compare, black holes are cemeteries and garbage collection stations. So what is in a black hole is simply the residual energy of the universe or the celestial bodies swallowed up by the black hole, such as compressed light and squeezed planets. Black holes account for 90% of the total mass of the universe, and black holes can be said to be everywhere. More specifically, we can find some evidence from the formation of black holes to explore what black holes in the universe are full of.
A black hole is a singular point with infinite density, infinite curvature of space-time height, infinite heat in the center and infinitesimal volume, surrounded by a part of empty sky, which is invisible. The process of a black hole is similar to that of a neutron star. A star is about to die, and its core rapidly contracts, collapses and explodes under its own gravity.
When all the core materials become neutrons, the contraction process stops immediately and is compressed into dense stars, and the inner space-time is also compressed. But in the case of a black hole, because the mass of the star core is so great that the contraction process goes on endlessly, the neutron itself is ground into powder under the attraction of its own squeezing gravity, and the rest is the matter with unimaginable density. Because of its great mass, anything near it will be sucked in by it. Black holes began to devour the shells of stars, but black holes could not devour so much matter. Black holes release some substances and emit two kinds of pure energy-gamma rays.
Because nothing can escape the attraction of black holes, including light, human beings can't observe black holes and other activities with the naked eye. Nowadays, people can use the X-ray telescope of Chandra Observatory to capture some information outside the black hole and try to understand it.