The goal is to find a single building block and a single force that could explain the matter and motion of the universe. The Standard Model has 12 particles (six quarks and six leptons) and four forces (electromagnetism, gravity, and the strong and weak nuclear forces). Furthermore, there is no known quantum theory of gravity (meaning our current definition covers just gravity involving things larger than, for example, common dust), so gravity isn’t even part of the Standard Model at all. So, physicists continue to look for an even more fundamental and underlying theory. To do that they need to reduce the number of both building blocks and forces.
Finding a smaller building block will be difficult, because that requires a more powerful particle accelerator than humans have ever built. The time horizon for a new accelerator facility coming on line is several decades and that facility will provide only a relatively modest incremental improvement over existing capabilities. So, scientists must instead speculate on what a smaller building block might look like. A popular idea is called superstring theory, which postulates that the smallest building block isn’t a particle, but rather a small and vibrating “string.” In the same way a cello string can play more than one note, the different patterns of vibrations are the different quarks and leptons. In this way, a single type of string could be the ultimate building block. [Top 5 Reasons We May Live in a Multiverse]
The problem is that there is no empirical evidence that superstrings actually exist. Further, the expected energy required to see them is called the Planck energy, which is a quadrillion (10 raised to the 15th power) times higher than we can currently generate.