Even after the annihilation of anti-matter and the formation of protons, neutrons and
electrons, the Universe is still a violent and extremely active environment. The photons
created by the matter/anti-matter annihilation epoch exist in vast numbers and have
energies at the x-ray level.
Radiation, in the form of photons, and matter, in the form of protons, neutrons and
electron, can interact by the process of scattering. Photons bounce off of elementary
particles, much like billiard balls. The energy of the photons is transfered to the matter
particles. The distance a photon can travel before hitting a matter particle is called the
mean free path.
Since matter and photons were in constant contact, their temperatures were the same, a
process called thermalization. Note also that the matter can not clump together by gravity.
The impacts by photons keep the matter particles apart and smoothly distributed.
The density and the temperature for the Universe continues to drop as it expands. At some
point about 15 minutes after the Big Bang, the temperature has dropped to the point where
ionization no longer takes places. Neutral atoms can form, atomic nuclei surround by
electron clouds. The number of free particles drops by a large fraction (all the protons,
neutrons and electron form atoms). And suddenly the photons are free to travel without
collisions, this is called decoupling.
The Universe becomes transparent at this point. Before this epoch, a photon couldn't
travel more that a few inches before a collision. So an observers line-of-sight was only a
few inches and the Universe was opaque, matter and radiation were coupled. This is the
transition from the radiation era to the matter era.
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