The huge imbalance between matter which is plentiful and anti-matter which is all but absent from the Universe is usually explained as follows: The Universe was created from the Big Bang in the Planck time of 10-43 seconds out of a spontaneous quantum fluctuation where neither space nor time existed before. The law of conservation of energy was not broken because the mass created exactly balanced the (negative) gravitational energy of that mass, and still does. Because the energy borrowed was zero, by Heisenbergs' Uncertainty Principle it can last indefinitely. Matter and anti-matter were created exactly equally, but due to some unknown mechanism perhaps involving baryon non-conservation and the decay of protons (baryons) into fermions (via X and Y particles created in profusion in the extremely hot early Universe), charge parity violation, and an exponential inflationary expansion of the Universe that was so fast as to evade thermal equilibrium, the exact balance between matter and anti-matter was very slightly altered by 2.5 parts in 109 towards an excess of matter. Soon afterwards the matter annihilated all the anti-matter leaving just the slight excess of matter. (This is the matter that we observe in the Universe today, but it represents but 4% of the measured mass of the Universe, the rest being spread between the unknown forms: dark matter and dark energy). The subsequent further expansive cooling allowed electrons and protons to form atoms producing a huge number of gamma ray photons. A large number of neutrinos were also produced in the fireball of the Big Bang, and these have continued to expand as the Universe expands, and may even be involved in the Universes expansion (see Accelerons)

The Universe over the next 13,700 Million years expanded and cooled, and with it cooled the gamma ray photons, which we now observe as the 2.7251 Kelvin cosmic microwave black-body background radiation visible from Earth and nearly uniform in all directions. The ratio of photons to baryons in the Universe today is in that same ratio, 2.5:109.

Superimposed on this black body radiation is a slight 3 mKelvin asymmetry in the temperature of the black body radiation in one direction, caused by Doppler shift as the Earth and Sun move through the frame of reference.