Here's why:

* The Big Bang: The CMB is a faint afterglow from the Big Bang, the event that created our universe. This radiation has been traveling for billions of years, expanding and cooling as the universe aged.

* Microwave Radiation: The CMB is primarily composed of microwaves, a type of electromagnetic radiation with wavelengths longer than visible light.

* Dominant Radiation: While there are other forms of radiation in the universe, the CMB is the most prevalent. It contributes the largest fraction of the total energy density of the universe.

While other forms of radiation, like visible light from stars and galaxies, are also present, the CMB dominates the overall radiation content of the universe.