The 3d structure of hadrons and origin of the proton's spin The Fundamental Pieces of Visible Matter: Offering an Unprecedented Insight into the 3D Structure of Hadrons and the Proton Spin Puzzle. All the visible matter in the universe is composed of atoms, which consist of electrons orbiting a small but extremely dense nucleus. The nucleus contains protons and neutrons, which also have a substructure of particles called quarks. Quarks come in different types, called flavors: up, down, strange, charm, bottom, and top. The quarks are held together by the strong nuclear force, which is mediated by other particles called gluons. Quarks and gluons, collectively called partons, are some of the most elementary pieces of visible matter in the universe. A “zoo” of particles, generically called hadrons, is made up of quarks and gluons.