The Standard Model of particle physics tells us what elementary particles make up matter in the universe. As far as we know, the particles in the Standard Model are the basic building blocks of matter and cannot be broken into smaller pieces, but physicists have not entirely ruled out the possibility that even smaller particles exist.
The idea of particle substructure is known as compositeness. Theories of compositeness say that the 12 matter particles and 12 antimatter particles described by the Standard Model are made up of even smaller units called preons. Different combinations of preons give rise to observable qualities such as mass and charge, allowing us to distinguish one composite particle from another.
If history is any indication, physicists could indeed discover that what we thought of as the most basic units of matter are actually made up of something smaller. The ancient Greeks came up with the idea of atoms, supposedly indivisible bits of matter. But research in the early twentieth century found that atoms are made up of negatively charged electrons surrounding a positively charged nucleus. Further experiments saw that the nucleus was made up of protons and neutrons, which in turn consist of quarks.
Detectors at the Large Hadron Collider (LHC) allow physicists to peer even further into the makeup of the smallest bits of matter. It will take many years of data collection and careful analysis to determine if compositeness exists. If it does, its discovery could open up a new world of subatomic particles. The unprecedented energy of proton collisions at the LHC could be what scientists need to find it.