What force(s) cause protons to be compacted in the nucleus of an atom?

Ok. All models of the atom show protons and neutrons compacted in the nucleus and this nucleus is surrounded by electrons in different quantum mechanical levels which have orbitals like s, p, d and f. Now in the ground state the atom is electrically neutral because it has the same number of protons and electrons. My question, or rather a student of mine's question, is " since all the protons have positive charges and like charges repel how can these protons be compacted in the nucleus of the atom?" Is it gravity? subatomic particle related or what?