The Lambda-CDM, Lambda cold dark matter, or ΛCDM model is the standard form of the Big Bang theory of cosmology governing the chronology of the universe. As implied by its name, the model postulates a cosmological constant, denoted by lambda (Λ), associated with dark energy, cold dark matter, denoted by CDM as well as ordinary matter. It is the simplest model that provides a reasonably good account of:the existence and structure of the cosmic microwave background; the large-scale structure in the distribution of galaxies; the observed abundances of hydrogen, helium, and lithium; the accelerating expansion of the universe observed in the light from distant galaxies and supernovae.