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Why is cosmic microwave background radiation found in the microwave region of the electromagnetic spectrum?

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Oval map of the cosmic microwave background radiation from NASA's WMAP mission, showing a mottled pattern of blue, green, yellow, and red representing temperature variations. — Cosmic microwave background A full-sky map produced by the Wilkinson Microwave Anisotropy Probe (WMAP) showing cosmic background radiation, a very uniform glow of microwaves emitted by the infant universe more than 13 billion years ago. Color differences indicate tiny fluctuations in the intensity of the radiation, a result of tiny variations in the density of matter in the early universe. According to inflation theory, these irregularities were the “seeds” that became the galaxies. WMAP's data support the big bang and inflation models, and cosmic microwave background is at the farthest limits of the observable universe.

The cosmic microwave background (CMB) is found in the microwave region of the electromagnetic spectrum due to the expansion of the universe over billions of years.12

Approximately 380,000 years after the big bang, the universe cooled sufficiently for protons and electrons to form neutral hydrogen atoms, a process known as recombination or decoupling. At this time, the universe became transparent to electromagnetic radiation, which initially had a blackbody spectrum corresponding to a temperature of about 3,000 K.341

As the universe continued to expand, this primordial light was stretched to longer, lower-energy wavelengths. This process, known as cosmological redshift, has shifted the original high-energy radiation into the microwave range, resulting in the current observed temperature of approximately 2.7 K. This shift serves as significant evidence for the expansion of the universe.3415