The Cosmic Microwave Background (CMB) is the faint afterglow of the Big Bang, representing the oldest light in the universe that we can observe. This thermal radiation emerged approximately 380,000 years after the Big Bang when the universe cooled to about 3,000 Kelvin, allowing electrons and protons to combine into neutral hydrogen atoms for the first time. This pivotal moment, called "recombination," made the previously opaque universe transparent, freeing photons to travel unimpeded through space.</p><p>Discovered accidentally in 1965 by Arno Penzias and Robert Wilson using a radio antenna, the CMB provided the first direct evidence supporting the Big Bang theory over competing models. Today, this radiation has cooled to just 2.725 Kelvin above absolute zero and appears as microwave radiation uniformly filling the entire sky.</p><p>The CMB's tiny temperature fluctuations—varying by only about 0.00001 degrees—serve as a cosmic blueprint, revealing the density variations that would eventually grow into galaxies and galaxy clusters. Missions like COBE, WMAP, and Planck have mapped these fluctuations with extraordinary precision, allowing cosmologists to determine that the universe is 13.8 billion years old and consists of approximately 5% ordinary matter, 27% dark matter, and 68% dark energy. The CMB remains our most powerful tool for understanding the universe's origin, evolution, and ultimate fate.