Black holes are not solid objects in the traditional sense. They are regions in spacetime where gravity is so strong that nothing, not even light, can escape. The boundary of this region is called the event horizon. Inside the event horizon, spacetime is so distorted that the laws of physics, as we understand them, break down.

Black holes are formed when a star collapses under its own gravity. When the star's core becomes dense enough, it creates a singularity, which is a point of infinite density and zero volume. The singularity is surrounded by the event horizon.

The physical composition of a black hole is not well understood. Some physicists believe that black holes are made up of a type of matter called "quark-gluon plasma." Quark-gluon plasma is a state of matter that exists at extremely high temperatures and densities. It is thought to have existed in the early universe, just after the Big Bang.

Other physicists believe that black holes are made up of a type of matter called "strange matter." Strange matter is a type of matter that contains strange quarks, which are one of the six types of quarks that make up protons and neutrons. Strange matter is thought to be very dense and stable, and it is possible that it could exist at the center of black holes.

The true nature of black holes is still a mystery, and it is one of the most active areas of research in physics.