This book shows how modern cosmology and astronomy have led to the need to introduce dark matter in the universe to account for mass. Some of this dark matter is in the familiar form of protons, electrons and neutrons, but most of it must have a more exotic form. The favored, but not the only, possibility is neutrinos of non-zero rest mass, pair-created in the hot big bang and surviving to the present day. After a review of modern cosmology, this book gives a detailed account of the author's recent theory in which these neutrinos decay into photons that are the main ionizing agents in hydrogen and nitrogen in the interstellar and intergalactic medium. This theory, though speculative, explains a number of rather different puzzling phenomena in astronomy and cosmology in a unified way and predicts values of various important quantities such as the mass of the decaying neutrino and the Hubble constant.