The cosmic microwave background (CMB) radiation is a remnant of the Big Bang and is often referred to as the "afterglow" of the early universe. It is composed of photons that have been traveling through the universe since they were emitted approximately 380,000 years after the Big Bang. These photons have been stretched and cooled over time, resulting in a microwave frequency range.
The wavelengths of the CMB photons are indeed stretched out, and they fall in the microwave part of the electromagnetic spectrum, with peak intensity around 1.9 mm (or equivalently, a frequency of about 160.2 GHz). Due to this stretching, the CMB radiation is considered to be in the radio spectrum.
Spectral lines, which are characteristic features of the emission or absorption of light by atoms or molecules, arise from transitions between different energy levels. In the case of the CMB radiation, the stretching of the wavelengths over cosmic time makes the spectral lines extremely difficult to detect. The original spectral features from the early universe, if they ever existed, would be significantly shifted and spread out across the microwave spectrum, making them essentially undetectable with current technology.
Furthermore, the CMB radiation is incredibly smooth and homogeneous on large scales, which means that any spectral lines would likely be obscured by the broad background radiation. The CMB spectrum itself follows a nearly perfect black body spectrum, with only tiny fluctuations that have been mapped and studied extensively.
To summarize, the wavelengths of the CMB radiation are indeed too stretched out to make individual spectral lines detectable with current technology. The CMB spectrum is mostly smooth and uniform, following a black body distribution with small fluctuations that provide valuable information about the early universe.