The standard model of particle physics forms the lowest level building blocks that we know. It contains three of the four fundamental forces (electromagnetic, and the strong and weak nuclear forces); only gravity is left out. These forces are created by the exchange of particles. All these particles fit on the T shirt.
How did the fundamental particles get these particular properties? Why do we have the elements that we do? The explanation for these facts lies in the early universe, and hence involves cosmology and Einstein's general theory of relativity. The explanation involves the idea of spontaneous symmetry breaking shortly after the Big Bang.
The crucial idea is spontaneous symmetry breaking. In the beginning, everything is massless and simple and the properties of the elementary particles come from "breaking" this highly symmetric situation as the universe expands. One obvious case, which is not really understood yet, is the breaking of the matter/antimater symmetry. Another is the Higgs Mechanism -- which involves a new particle that is supposed to explain the origin of mass. See here for a nice story on how this cocktail party analogy came to be. The Higgs boson is one of the main things being looked for at the LHC. The latest info on the search at ATLAS is here. More on the Higgs mechanism here.
Here is a little cartoon of how things are supposed to have fallen out after the Big Bang.
Tablet notes: [Lec 8] [Lec 8.5] [Lec 9][Lec 10].