Will the universe continue to expand forever, reverse its expansion and begin to contract, or reach a delicately poised state where it simply persists forever? The answer depends on the amount and properties of matter in the universe, and that has given rise to one of the great paradoxes of modern cosmology: there is too little visible matter to account for the behavior we can see. Over ninety percent of the universe consists of "missing mass" or "dark matter" - what Lawrence Krauss, in his classic book, termed "the fifth essence."In this new edition of The Fifth Essence, retitled...