Cosmic microwave background
Cosmic microwave background (CMB) radiation describes the electromagnetic waves that propagate through our entire universe.
The usual explanation of the CMB radiation trotted out by atheistic scientists is that it is 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.
The scientific version of events doesn't make sense when viewed from a Christian perspective, as it requires the universe be at least 380,000 years old for the transition to have occurred. However, scholarly analysis of the Bible indicates that the universe is around 6000 years old, which is backed up by many observations in many fields from geology to astronomy.
A possible explanation for the CMB is that it is the light (Genesis 1:2) from the moment of the Creation around the universe. If the Lord had suddenly created the universe and flooded it with perfectly uniform light (electromagnetic waves), we would indeed see the remnants to this day, except for minuscule variations introduced by a fraction of the light being blocked by the Earth (which of course preceded the light, Genesis 1:1).
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 ), 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.