Inside a black hole, our current understanding based on general relativity suggests that there is a region called the singularity, which is a point of infinite density and infinite curvature. However, our knowledge of what exactly occurs at the singularity is limited because our current theories, including general relativity, break down under such extreme conditions.
According to general relativity, the singularity is thought to be surrounded by an event horizon, which is a boundary beyond which nothing, including light, can escape the gravitational pull of the black hole. This is why we cannot directly observe or gather information from inside a black hole.
Despite the inability to directly observe the interior of a black hole, scientists have gained knowledge about them through indirect means. For example, the effects of black holes on their surroundings, such as the gravitational influence on nearby matter and the emission of radiation from the surrounding material falling into the black hole, provide valuable insights. Additionally, mathematical models and theoretical physics help us understand the behavior of black holes based on our current understanding of gravity and quantum mechanics.
It's important to note that our understanding of black holes is still an active area of research, and there are ongoing efforts to develop a theory that can unify general relativity and quantum mechanics, which could potentially shed more light on the nature of black holes and their interiors.