Spectacular observational breakthroughs by recent experiments, and particularly the WMAP satellite, have heralded a new epoch of CMB science forty years after its original discovery. Taking a physical approach, the authors probe the problem of the 'darkness' of the Universe: the origin and evolution of dark energy and matter in the cosmos. Starting with the observational background of modern cosmology, they provide an up-to-date and accessible review of this fascinating yet complex subject. Topics discussed include the kinetics of the electromagnetic radiation in the Universe, the ionization history of cosmic plasmas, the origin of primordial perturbations in light of the inflation paradigm, and the formation of anisotropy and polarization of the CMB. This timely and accessible review will be valuable to advanced students and researchers in cosmology. The text highlights the progress made by recent experiments, including the WMAP satellite, and looks ahead to future CMB experiments.