Extragalactic background light in the context of "Mean free path"

Cosmic background radiation is electromagnetic radiation that fills all space. The origin of this radiation depends on the region of the spectrum that is observed. One component is the cosmic microwave background. This component is redshifted photons that have freely streamed from an epoch when the Universe became transparent for the first time to radiation. Its discovery and detailed observations of its properties are considered one of the major confirmations of the Big Bang. Background radiation is largely homogeneous and isotropic. A slight detectable anisotropy is present which correlates to galaxy filaments and voids. The discovery (by chance in 1965) of the cosmic background radiation suggests that the early universe was dominated by a radiation field, a field of extremely high temperature and pressure.

There is background radiation observed across all wavelength regimes, peaking in microwave, but also notable in infrared and X-ray regimes. Fluctuations in cosmic background radiation across regimes create parameters for the amount of baryonic matter in the universe. See cosmic infrared background and X-ray background. See also cosmic neutrino background and extragalactic background light.