According to Big Bang theory and modern nuclear physics, the universe may once have had equal quantities of each, however as the universe expanded and cooled, this symmetry was broken. This could have happened because it is already known that the weak interaction which causes some particles to decay favors matter over anti-matter as the end products. The cosmic background radiation is believed to be all that is left of this symmetric state because there are about 10 billion photons for every matter quark. This means that matter was favored over anti-matter by 1 part in 10 billion, and after the epoch of annihilation ended, all that was left was the 1 quark of matter for each of the 10 billion pairs of matter and anti-matter quarks than annihilated.