 Cosmic microwave background

The Cosmic Microwave Background (CMB) is the afterglow radiation from the Big Bang, providing crucial evidence for the Big Bang theory and our understanding of the early universe. It is a faint, uniform background radiation that fills the universe and can be detected in every direction in space.

If you point a light detector to any empty area of the sky, you will still get some light.

The existence of this is quite mind blowing, since "there is nothing there emitting that light".

To make sense of how it is possible to see this light, you can think of the universe as the expanding raisin bread model, but it expands faster than light (thus the existence of the cosmological event horizon), so we are still receiving light form the middle, not the borders.

CMB is basically perfectly black-body radiation at 2.725 48 K, but it has small variations with variations of the order of 200 microKelvin: cosmic microwave background anisotropy.