Minute temperature fluctuations (anisotropies) in the CMB are the 'seeds' for all large-scale structure in the universe.

While remarkably uniform, the CMB is not perfectly smooth. Satellite missions like COBE, WMAP, and Planck have detected tiny temperature variations — anisotropies — in the CMB, on the order of just a few tens of microkelvin (about 1 part in 100,000). These slight differences in temperature correspond to regions of slightly higher or lower density in the early universe, long before galaxies or stars formed. These density fluctuations were gravitationally unstable: denser regions had a slightly stronger gravitational pull, attracting more matter over time. Over billions of years, these tiny primordial seeds of density grew, eventually collapsing to form the first stars, then galaxies, and finally the vast cosmic web of galaxy clusters and superclusters we observe today. The precise pattern and amplitude of these anisotropies, meticulously mapped by these missions, provide crucial data for testing and refining the standard model of cosmology, including the amounts of dark matter and dark energy, and the properties of the early universe.