This controversial book examines one of the most fundamental questions of modern cosmology: how much matter is there in the Universe? This issue affects theories of the origin and evolution of the Universe as well as its geometrical structure and ultimate fate. The authors address this debate and point out the most likely avenues for determining the actual density of Universe matter in both visible and invisible forms by pulling together evidence from all available sources. They conclude that the balance of arguments presently lies with a density of around twenty percent of the critical density required for the Universe to ultimately recollapse. Written by two eminent cosmologists, this topical and provocative book will be essential reading for all cosmologists and astrophysicists.