What is the smallest known unit of matter?
Matter is made up of molecules, which are made up of various atoms. The water molecule is made up of three atoms, two hydrogen atoms and one oxygen atom. Atoms themselves are made up of neutrons, protons and electrons. Protons and neutrons are made up of quarks and gluons, quarks are the building blocks of matter and they are held together by gluons. Electrons are made from leptons, leptons are a type of fermion, quarks are another type of fermion. Bosons are particles that produce and carry forces and include photons and gluons.
This chart of the Standard Model organizes all that I’ve mentioned above with some more detail.