The concept of what lies at the bottom of a black hole is still a subject of active scientific research and debate. According to our current understanding of black holes based on general relativity, the region at the center of a black hole is known as the singularity.
A singularity is a point of infinite density and curvature where the laws of physics, as we currently understand them, break down. It is a region where the gravitational pull becomes extremely strong, leading to a distortion of spacetime. At the singularity, the matter and energy that have collapsed into the black hole are compressed to an infinitely small and infinitely dense state.
Due to the extreme conditions within the singularity, our current understanding of physics cannot accurately describe what occurs. It is often said that our current theories, such as general relativity, are incomplete when it comes to describing the behavior of singularities. In this regime, the effects of quantum mechanics are expected to become significant, but a complete theory of quantum gravity that can adequately describe what happens inside a singularity is still an area of active research.
It is worth noting that our understanding is based on classical general relativity, and future advancements in physics may provide new insights and refinements to our understanding of black holes and their singularities.