According to our current understanding in particle physics, fundamental particles are considered to be indivisible and have no known substructure. These particles are point-like entities, meaning they have no size or internal structure. Examples of fundamental particles include electrons, quarks, and neutrinos.
However, it's important to note that our understanding of the universe is continually evolving, and new discoveries may challenge or expand our current knowledge. The search for deeper levels of fundamental particles and substructures is an active area of research.
One possibility for smaller entities beyond fundamental particles is the concept of "strings" in string theory. String theory proposes that the fundamental building blocks of the universe are tiny, vibrating strings, rather than point-like particles. These strings are believed to be much smaller than the smallest length scales currently probed by experiments. However, it's worth noting that string theory is still a highly theoretical and speculative framework, and it has not yet been confirmed through experimental observation.
Another avenue of exploration is the study of quantum fluctuations and virtual particles. According to quantum field theory, the vacuum of space is not empty but is seething with quantum fluctuations. These fluctuations can manifest as virtual particles that spontaneously pop in and out of existence. While these virtual particles are not considered to be "smaller" than fundamental particles, they do arise from the underlying quantum nature of the universe and can have observable effects.
To summarize, according to our current understanding, fundamental particles are considered to be the smallest entities in the universe. However, ongoing research and theoretical investigations, such as in string theory, aim to uncover deeper levels of reality and explore the possibility of substructures beyond our current understanding.