The cosmic microwave background (CMB) is the cooled remnant of the radiation released at photon decoupling. It is an almost perfect blackbody with temperature about $T \approx 2.725\,\mathrm{K}$ today. Tiny temperature fluctuations of order $\Delta T/T \sim 10^{-5}$ encode the seeds of all later structure.

The angular power spectrum of the CMB contains acoustic peaks from oscillations in the photon-baryon fluid before decoupling. The locations and heights of these peaks depend on the baryon density, dark matter density, curvature, and other parameters. This makes the CMB a cornerstone dataset for parameter inference in cosmology.