Quantum gravity is a field of theoretical physics that seeks to unify the theories of general relativity (which describes gravity on a large scale, such as planets and galaxies) with quantum mechanics (which describes the behavior of matter and energy on very small scales, such as atoms and subatomic particles). The unification of these two theories is necessary to understand the behavior of matter and space-time in extreme conditions, such as near black holes or during the early moments of the universe.

Currently, general relativity and quantum mechanics are highly successful theories in their respective domains, but they are fundamentally incompatible with each other when applied together in certain contexts, particularly at extremely small distances and high energies, such as those present in the early universe or at the center of black holes. In these regimes, the effects of both gravity and quantum mechanics become significant, leading to theoretical inconsistencies.

Several approaches have been proposed to reconcile general relativity and quantum mechanics, such as string theory, loop quantum gravity, causal dynamical triangulation, and others. Each approach offers a different perspective on how space-time and matter might behave at the quantum level, but as of now, none has been experimentally confirmed, and the quest for a complete theory of quantum gravity remains one of the major challenges in theoretical physics.