The Young Universe presents four major physical and astrophysical themes related to these extreme phases of the primordial universe. In particular, it presents the physics of the primordial plasma and the concepts of quantum and particle physics necessary to describe this extreme state. It discusses the cosmological background radiation and explores inflation, an extremely rapid expansion phase that is believed to have occurred very early in cosmological history and to have shaped our present universe. The book also provides a synthesis of the dark matter problem.