Everything happened very quickly.

The incredibly rapid expansion of the universe progressed through a series of stages, or eras, during which the forces that bind matter separated from one another, and photons of light collided to form particles of matter. The greatest changes in energy and matter occurred during the first few seconds.

The first era was the heavy-particle era, which lasted from 10^-44 to 10^-4 seconds, an amazingly short time. During this era, pairs of particles, matter and antimatter, were created from colliding photons of light. The universe could be described as a smooth soup of high-energy, light and massive elementary particles. Neither atoms nor the nuclei of atoms existed.

Next came the light-particle era, from 10^-4 to 10 seconds. Much of the energy of the infant universe had been converted to matter, and only enough remained to produce lighter particles, such as electrons and positrons. Protons and electrons interacted to form neutrons.