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Protons and neutrons, packed together. All of the mass, none of the room.
Rutherford fired alpha particles at gold foil in 1909, expecting them all to pass through. A few came straight back. That was this.
Two up quarks, one down, and the gluon field that will not let them leave. A proton is less an object than a knot in a field. Its size is the width of the knot, not the size of anything inside it.