Discovered in 1965 by Penzias and Wilson, the cosmic microwave background (CMB) is relic radiation from 380,000 years after the Big Bang. As the universe cooled, photons decoupled from matter, creating a snapshot of the infant universe. Tiny fluctuations in the CMB reveal the seeds of galaxies and large-scale structures. Observing the CMB helps test cosmological models, including inflation and dark matter composition. The uniformity and subtle irregularities of the CMB encode the universe’s early dynamics. It’s literally the oldest light we can detect.
It matters because the CMB confirms the Big Bang theory and informs cosmic evolution models.
It also provides a baseline for understanding the universe’s composition, including dark matter and dark energy.
The universe’s first light still travels across space, billions of years later.
NASA [nasa.gov]