Different combinations of up-quarks and down-quarks make protons and neutrons. Everything you see appears to be made of up-quarks, down-quarks, and electrons. However, there is evidence that the universe consists of more particles. A fourth fundamental particle called a neutrino was discovered in the 50’s. These particles hardly ever interact with other matter, so their presence is difficult to detect. Up-quarks, down-quarks, electrons, and neutrinos make up one family of particles.

Two other families of particles exist, each having analogous particles to the first family. The major difference between the families is that the particles of family 2 are more massive than particles of family 1, and the particles of family 3 are more massive than family 2. Most particles from family 2 and 3 exist ephemerally, and are only seen through high-energy collisions in particle-accelerators. It is possible that such particles have not existed since the big bang except through high energy collisions in labs.

Each particle from each family also has an antiparticle. These antiparticles are identical in mass, but opposite in other respects. When a particle collides with its antiparticle they annihilate each other producing pure energy. This is the reason antimatter is only seen in laboratories. It is possible that other galaxies of the universe are composed entirely of antimatter.

Though we can verify the existence of all these particles, we don’t know many “whys.” For instance, why 3 families? Why not 4, or 6, or just 1? Why does each particle have a seemingly arbitrary mass? Why not some other number? String theory offers plausible explanations for these questions.