The Cosmic Microwave Background

At Microwave wavelengths the sky is glowing with almost equal brightness in all directions.

The CMB light rays which we are receiving now set off in straight lines 14 billion years ago when the plasma combined to form neutral atoms.
So they started from a spherical "shell" and have crossed 14 billion light years of space.
Started off as visible light - stretched with space all the way down to microwaves.

Variations in CMB brightness tell us about the (small) variations in the density of the plasma at that time...