The origin of the Universe is always a difficult and stimulating question in astronomy. Around 1930s, astronomers discovered that galaxies are going away from us. The further away it is, the faster it goes away. This leads us to conjecture that the Universe is expanding. This is the basic idea of the Big Bang Theory in cosmology. If we play this cosmic "movie" backwards, the galaxies will come towards us. The density and temperature of the Universe will become higher and higher. At some point, even the atoms will be broken apart. To understand what will happen before that, we have to study the constituents of atoms: the fundamental particles.

Depends on different counting methods, there are about 18 types of fundamental particles. Twelve of them are the building blocks of matter, and the rest mediates the interactions between particles.

These twelve particles form matter because they follow the Pauli's exclusion principle. This principle says, among other things, that particles of the same type cannot occupy the same point in space and thus they hate each others. If you put together a large amount of particles, they can only sit next to each other and take up some space. This is what we think of an object.

Only three of them are important in everyday life: u, d and e. Materials are made up of atoms, and the outer part of an atom consists of one or more electrons. The nucleus consists of protons and neutrons. Proton is, in turn, made up of two up quarks and one down quark (uud). Neutron is made up of two down quarks and one up quark (udd). In summary, the up and down quarks make up all the nuclei and electrons orbiting around nuclei form atoms.

The other particles are discovered in some high energy experiments. They are all important in astronomy. For example, the core of the Sun generates energy, as well as electron neutrinos. However, the amount of neutrinos we detected is only about one third of what we expected. Astronomers still do not know the resolution. This is the solar neutrino problem.

The remaining six types of particles do not follow the Pauli's exclusion principle. They can join forces and add up their effects, become the mediators of interactions. For example, photon (g) mediates electromagnetic interaction. One charged particle feels the attraction or repulsion of another charged particle because there are many invisible photons going back and forth between the two charged particles to transmit the effect. Similarly, gluons transmit strong nuclear interaction. W^+, W^- and Z⁰ transmit weak nuclear interaction. Gravitons transmit gravitational interaction. Except graviton, all others particles have been found in experiments.

All these 18 types of particles are fundamental in the sense that up to the length scale of 10^-17m, which is our present experimental limit, we could not find any structure of them. Each of them also has its own anti-particle with similar but opposite properties.

There are other particles proposed to exist, but have not been found in experiment. Magnetic monopole is one of them. If the theory is correct, magnetic monopole not only exist, a lot of them should have been created at the beginning of the Universe. Why cannot we find one is still a mystery in cosmology. On the other hand, dark matter is omnipresent in galaxies, but we know almost nothing about its nature. One proposal is that it is some kind of weakly interacting massive particle (WIMP). Is dark matter really some exotic fundamental particle? We need more observations and experiments to answer this question.