Cosmic microwave background
Cosmic microwave background (CMB) radiation describes the electromagnetic waves that propagate through our entire universe. It is thought to be left over radiation from the big bang, namely the radiation scattering off of the opaque, dense plasma of the early universe just before the transition to the transparent universe we observe today. This transition, called "last scattering", is believed to have occurred approximately 380,000 years after the big bang.
One interesting feature of the CMB is that it provides a rest frame against which one can measure the motion of galaxies and other astronomical objects. Since the cosmic microwave background is made of electromagnetic waves, a moving observer will observe a Doppler shift of the cosmic microwave background, whereas a stationary observer will not. Since the frequency of the cosmic microwave background is known (160.2 GHz [1]), the velocity of the observer can easily be calculated. Our very Earth sees a Doppler shift in the cosmic microwave background, a shift that must be subtracted from measurements of the CMB to obtain its true temperature distribution.
The thermal black-body spectrum of the cosmic microwave background is 2.725 K. [1]