Physicists often remark about the absence of antimatter in the universe. My question here sounds simple, but the answer has proved elusive to me. Perhaps someone here can shed some light. The question is: What is the physical property that "matter" has that distinguishes it from "anti-matter"? The universe around us is made up of what we call "matter" which is essentially a couple of quark types (u and d) and a lepton type (e). So my question can be rephrased as: "What physical property does the lepton 'e' share with the quarks 'u' and 'd' that gets them all in the same group (matter) and their anti particles classed as anti-matter?" Particle Physics says we have 24 fermions (spin half particles). Each fermion has an anti-fermion. Each fermion also has its weak isopsin partner (the particle you get if you add or remove a W+/- as appropriate. Eg u->d when you emit a W+ or e->neutrino when you emit a W-. etc.). But that doesn't say which of the fermions is matter and which is anti-matter. The fermions are split into those that experience the strong force (quarks and anti quarks) and those that don't (leptons and anti-leptons). The grouping of the electron with the up and down quarks into "matter" spans this. But what is the property? The Standard Model of Electro -Weak interactions says that only left handed fermions and right handed anti-fermions experience the weak force. I've heard this used as an arguement to group left handed electrons and left handed up and down quarks etc. However theory applies to zero mass particles. I can accept that this can be used in the lepton regime where the neutrinos are effectively massless so even though the charged leptons are massive there is one component -the neutrino- which is massless. However in the quark regime all the fermions have mass. So can anyone explain to me what is the property that the electron and its associated leptons have that gets them grouped with the 'up' quark rather than the anti-up quark? Other than its statistically large representation within the observed universe, which is not a property of the particle itself.