The ancient Greeks believed atoms were the smallest bits of matter in the universe. Then scientists in the 20th century split the atom, yielding tinier ingredients: protons, neutrons and electrons. Protons and neutrons, in turn, were shown to consist of smaller particles called quarks, bound together by “sticky” particles, the appropriately named gluons. These particles, we now know, are truly fundamental, but even this picture turns out to be incomplete.