Final answer:
The "something-besides-just-the-proton" is a neutron, which together with protons, makes up the nucleons in the nucleus of an atom. Neutrons are neutral and have a similar mass to protons, which are positively charged. Quarks are the smaller particles that constitute protons and neutrons.
Step-by-step explanation:
The "something-besides-just-the-proton" referred to in the question is the neutron. The nucleus of an atom consists of two main types of particles: protons and neutrons, collectively known as nucleons. Protons carry a positive charge, while neutrons are neutral particles.
Both nucleons have nearly the same mass and are much more massive than electrons. Quarks are even more fundamental particles that make up protons and neutrons. In the original experiments where quarks were discovered, scientists scattered high-energy electrons off of protons and observed patterns suggesting the presence of these smaller, dense particles.