Cosmic microwave background
Cosmic microwave background radiation describes the electromagnetic waves that propagate through our entire universe. It is thought to be left over radiation from the Big Bang, but its true origin is unknown.
Some believe that the cosmic radiation background contradicts relativity; it provides a way to irrefutably define what is moving and what is not moving. Since the cosmic radiation background is made of electromagnetic waves, a moving observer will observe a Doppler shift in the cosmic radiation background, whereas a stationary observer will not. Since the frequency of the cosmic background radiation 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.