Note that both quarks and leptons exist in three distinct sets. Each set of quark and lepton charge types is called a generation of matter (charges +2/3, -1/3, 0, and -1 as you go down each generation). The generations are organized by increasing mass.

All visible matter in the universe is made from the first generation of matter particles -- up quarks, down quarks, and electrons. This is because all second and third generation particles are unstable and quickly decay into stable first generation particles.

Wait a minute. If the higher generations of matter decay quickly, are rarely observed, and do not make up any of the stable matter around us, why do they exist at all?

Good question. In fact, when the muon was discovered physicist I.I. Rabi asked,

So why do we have generations of matter at all? Why three of them? We don't know. And without understanding why the second and third generation particles exist, we cannot rule out the possibility that there are yet more quarks and leptons that we have not discovered yet. Or perhaps the answer is that quarks and leptons aren't fundamental, but are made up of even more elementary particles whose composite particles we observe as quarks.