What is the Cosmic Microwave Backround (CMB)?

 

Temperature map of the cosmic microwave backround by the Planck spacecraft

The cosmic microwave background is leftover radiation from the Big Bang, or the time when the universe began, representing its afterglow. It's essentially the oldest light we can detect.

According to the standard model of cosmology, our universe began its journey in an extremely hot and dense state, with very low entropy.[1] After the Big Bang, it began an expansion that has never stopped, becoming less dense and cooler.

Evolution of the universe

Therefore, in the first moments after the Big Bang, the primordial universe was filled with an opaque, hot plasma of subatomic particles. As space expanded, the matter and energy in the universe also expanded and cooled. After roughly 380.000 years after the Big Bang, the temperature had dropped enough and the photons [2] freed themselves from the hot plasma and began their journey. However, they grew less energetic. From the highest energy, meaning gamma radiation, they were reduced to radio waves, with the lowest energy. This is what we see in the CMB.

The colors of the map represent very small temperature differences, which can be interpreted as differences in the density of the primordial hot plasma that ultimately resulted in the formation of the galaxies we see today.[3]

[1] entropy = the number of states a system can have, associated with disorder ( high entropy means high disorder and low entropy means order )

[2] photon = the fundamental particle of light ( gamma rays, visible light, microwaves, etc. ) 

[3] The denser space attracted the matter from the less-denser space. Galaxies started to grow from those denser pockets of space.