Six quarks, six leptons, together with the gluons of QCD and the photon and weak bosons, are enough to describe the tangible world and more, with remarkable economy. Only the Higgs boson is missing among the ingredients of the canonical Standard Model. And yet we know we are missing much more than this. The last ten years of cosmological observations have established that the ordinary matter of quarks and leptons accounts for just 5% of the energy density of the Universe, that another 23% is “dark matter,” outside the Standard Model, and 72% of the energy density isn't due to matter at all. Moreover, we can't answer the most basic question of all: why is there something rather than nothing? Why didn't all the matter created in the Big Bang ultimately annihilate, particle against antiparticle? Andrei Sakharov explained that CP violation must be part of the answer, but we know it isn't just the CP violation of the CKM matrix, for that wouldn't account for the amount of matter that remains. On the other hand, the strong interactions might have been CP violating but aren't. Why not?

These questions are pressed upon us by facts and demand answers. Other questions arise more from aesthetics: Are the strong and electroweak forces themselves unified? What about gravity? Are there more forces still to be discovered? Why are there three generations of quarks and leptons? Even more audaciously, why are there three spatial dimensions, or perhaps, are there more than three spatial dimensions?