And so, ad infinitum: Smallest of the small
Scientists are searching for particles smaller than the familiar ones of the Standard Model.
Big fleas have little fleas,
Upon their backs to bite 'em,
And little fleas have lesser fleas,
and so, ad infinitum. —Augustus De Morgan
Suppose you were the size of the universe - you would see a bunch of luminous, point-like dots surrounding you. If you shrank, you'd identify these points as galaxies, each with a rich structure and each filled with a new bunch of point-like dots. As you shrink more, you'd see that these new dots were not points, but stars.
If you pressed the supershrink button on your shrink ray, you'd zoom down through the size of humans, through cells, molecules, atoms, atomic nuclei, protons and neutrons and finally down to the size of the quarks and leptons that constitute the knowledge frontier of the super small. Over the past hundred years or so, a series of objects like atoms were thought to be the smallest building block of matter, only to have subsequent research show that we were wrong.
The Standard Model exemplifies the micro world of particle physics. Two classes of particles, called quarks and leptons, are thought to be the smallest building blocks of the universe. They have no size and contain nothing in them, but, if mixed correctly, they can build atoms, us, the entire universe.
After about a century of progressively finding something smaller than what we thought was the smallest, why should anyone think that these quarks and leptons are the final word? The answer is complex.