Today, we know beyond a shadow of a doubt that the CMB exhibits a blackbody spectrum. The peak wavelength of the background radiation is This is in the short-wavelength part of the radio, also known as the microwave part of the EM spectrum. Thus, the background radiation has come to be known as the Cosmic Microwave Background radiation, or CMB for short. In addition, the laws that govern the propagation of photons in an expanding universe tell us that the early universe was also hot. Hydrogen atoms were ionized then.
Photons interact very strongly with free electrons.