The Cosmic Microwave Background (CMB) provides strong evidence for dark matter's existence and its precise cosmological abundance.

The Cosmic Microwave Background is the faint afterglow of the Big Bang, a snapshot of the universe when it was only about 380,000 years old. Tiny temperature fluctuations, or anisotropies, in the CMB map reveal the seeds from which today's large-scale cosmic structures (galaxies and galaxy clusters) grew. The specific patterns and scales of these fluctuations are highly sensitive to the overall composition of the universe, including the density of both ordinary ('baryonic') matter and dark matter. Data from missions like NASA's WMAP and the European Space Agency's Planck satellite have meticulously measured these CMB anisotropies. Cosmological models that include dark matter perfectly match these observed patterns, indicating that dark matter makes up about 26.8% of the universe's total mass-energy density, significantly more than the 4.9% attributed to ordinary matter. Without dark matter, the CMB data simply cannot be explained.