Quantum mechanics is currently considered the fundamental theory that describes the behavior of matter and energy at the smallest scales. It provides a powerful framework for understanding and predicting the behavior of particles, atoms, and molecules. However, it's important to note that our understanding of the universe is always evolving, and new discoveries may lead to deeper levels of understanding.
At present, the notion of a smaller level than quantum mechanics is not well-established in mainstream physics. Quantum mechanics is incredibly successful in describing the behavior of particles and fields, and it has been rigorously tested and confirmed through numerous experiments. It forms the foundation of various fields, including quantum field theory, quantum electrodynamics, and quantum chromodynamics.
It is worth mentioning that certain theories, such as string theory, propose the existence of fundamental entities known as strings or branes, which are smaller than the scale typically addressed by quantum mechanics. However, these theories are still highly speculative and have not yet been experimentally confirmed.
Exploring physics at scales smaller than quantum mechanics would require a theory that goes beyond our current understanding. Such a theory, often referred to as a "theory of everything" or "quantum gravity," would need to reconcile quantum mechanics with the principles of general relativity, which describes gravity at large scales. Developing a consistent theory of quantum gravity remains an active area of research in theoretical physics.
While we don't currently have a well-established understanding of a level smaller than quantum mechanics, ongoing scientific research and future breakthroughs may provide us with deeper insights into the nature of the universe.