A black hole is a celestial body that exists in the universe according to the modern general theory of relativity. Its gravitational pull is so strong that the escape velocity within its event horizon exceeds the speed of light.

Black holes are among the most captivating and enigmatic entities in the universe. Human exploration of black holes can be traced back to 104 years ago when the astronomer Schwarzschild derived a solution to Einstein's general relativity field equations during his calculations.

This solution revealed that if the radius of a static star is smaller than a certain value, the star will possess a boundary. Once an object crosses this boundary, not even light can escape. American physicist John Wheeler coined the term "black hole" to describe this remarkable celestial object.

Compared to other celestial bodies, black holes exhibit extraordinary characteristics. They cannot be observed directly, compelling scientists to rely on conjecture regarding their internal structure. The concealment of black holes arises from the curvature of spacetime, which is dictated by the presence of a gravitational field.

In accordance with general relativity, spacetime becomes curved in the presence of a massive object. Consequently, light traveling through the vicinity of a dense celestial body is deflected from its original path due to this curvature.

The formation of black holes depends on their type and origin. To date, scientists have identified at least four distinct types: microscopic black holes, stellar black holes, intermediate-sized black holes, and supermassive black holes.

Microscopic black holes, theorized to be as small as atoms, are thought to have formed during the earliest stages of the universe's existence. While these minuscule black holes have remained purely hypothetical, they are envisioned as tiny dark vortices dispersed throughout the cosmos, collectively possessing a mass hundreds of times that of the sun.

Stellar black holes emerge when a star depletes its nuclear fuel and undergoes gravitational collapse. Typically, the mass of a stellar black hole ranges from several to tens of times that of the sun.

The mechanism behind the formation of intermediate-sized black holes is not yet fully understood. However, these black holes have masses ranging from hundreds to tens of thousands of solar masses.

On the other hand, supermassive black holes, reaching millions or billions of solar masses, are the largest known black holes and are believed to exist at the centers of galaxies.

Regarding the internal structure of black holes, current theories propose the presence of a singularity—a point with infinite mass and infinitesimal volume.

Surrounding the singularity is the event horizon, which marks the boundary of a black hole and cannot be observed. Inside the event horizon, spacetime becomes extraordinarily warped, and gravity is immensely strong.

Scientists are continuously striving to comprehend the nature and internal structure of black holes. Various theories have been proposed, exploring concepts such as the characteristics of singularity and the potential existence of a "wall of fire."

However, due to the unique properties of black holes, direct observation and verification of these theories remain elusive. Consequently, black holes persist as an exceptionally challenging and mysterious subject of study in the realm of the universe.