This radiation is regarded as the best available evidence of the Big Bang theory -- it gives a snapshot of the Universe when the temperature dropped enough to allow electrons? and protons? to form hydrogen atoms, thus making the universe transparent to radiation. When it originated some 300,000 years after the Big Bang -- this point in time is generally known as the "last scattering surface" -- the temperature of the Universe was about 6000 K. Since then it has dropped because of the expansion of the Universe, which cools radiation inversely proportional to the fourth power of the Universe's [scale length]?.
One of the microwave background's most salient features is a high degree of isotropy. There are some anisotropies, the most pronounced of which is the dipole anisotropy at a level of about 10-4 at a scale of 180 degrees of arc. It is due to the motion of the observer against the CBR, which is some 700 km/s for the Earth.
Much smaller variations due to external physics also exist; the Sunyaev-Zel'dovic-Effect? is one of the major factors here. Even more interesting are anisotropies at a level of roughly 1/100000 and on a scale of a few arcminutes. Those very small variations correspond to the density fluctuations at the last scattering surface and give valuable information about the seeds for the large scale structures we observe now.
The CBR was predicted by Murray Gell-Mann? in the 1940s and was independently discovered in the 1950s by Penzias and Wilson, who received a Nobel Prize for this discovery. Since the cosmic microwave radiation is rather difficult to observe with ground-based instruments, CMB research makes increasing use of space-borne experiments. Probably still the most famous of these is the Cosmic Background Explorer (COBE?) that was flown in 1992-1996.
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