I think this is a simpler and more complete explanation: "The cosmic microwave background (CMB) is an almost-uniform background of radio waves that fill the universe" [1]
It's the quantum radiation finger print left over from when the entire universe was smaller than a billionth of a proton and about to get very much bigger almost instantaneously. (the poetic version)
Just light from distant galaxies? Light is moving at C, so it cannot age because it experiences no time, but it's not always the case.
Light must age, otherwise our infinite Universe, which exists for infinity, will produce infinite^2 amount of light, which is not possible.
Current explanation of light aging is stretching of space in all directions. However, same effect can be explained by local random stretching of space when, for example, virtual pair of quantum particles emerges and then disappears. Casimir effect is pretty strong, which indicates that our space is very bumpy, full of white noise.
[1] http://cosmology.berkeley.edu/Education/CosmologyEssays/The_...