Spectacular observational breakthroughs, particularly by the WMAP satellite, have led to a new epoch of CMB science long after its original discovery. Taking a physical approach, the authors of this volume, which was first published in 2006, 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 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 plamas, the origin of primordial perturbations in light of the inflation paradigm, and the formation of anisotropy and polarization of the CMB. This fascinating review will be valuable to advanced students and researchers in cosmology.