Chapter 10. In Search of the Missing Mass

...in the early Universe, this particular ‘pressure’ does not involve the massive particles, such as protons, but only the light particles of the relativistic ‘gas’. These include photons, electrons and positrons, and the three types of neutrino and their antineutrino counterparts.
We owe our existence to a very tiny imbalance in the laws of physics, a preference for matter over antimatter in the decay of X bosons that amounts to no more than one extra quark for every billion antiquarks—an imbalance equivalent to one ten millionth of one per cent of all the matter that existed in the form of X and anti-X pairs in the GUT era.