One of the great unsolved mysteries of our time is that astronomers can only see a small amount of the matter that makes up our universe. We know that the missing matter exists because of is influence on the stars and galaxies we can see. But we just can't find it. The search for this so-called dark matter is on. Whatever it is, it is quite unlike the stuff that atoms, planets, stars and people are made of. Then the mystery deepens; the cosmic mix also appears to include an extra ingredient, an intangible commodity called dark energy that is causing the expansion of the universe to accelerate. Evidence suggests that dark energy makes up more than 70 percent of the overall content of the universe. Matter itself, and gravity too, plays second fiddle to dark energy. The fate of the universe depends on it: it may continue to expand forever, it may tear itself apart in a 'Big Rip', or it may even collapse in on itself in a terminal 'Big Crunch'. The Dark Side of the Universe paints an indepth, detailed picture of this extraordinary new view of the cosmos. It highlights the key discoveries, explains the underlying concepts, and examines current thinking about the nature of dark matter and dark energy. It looks, too, at alternative ideas which challenge the very existence of dark matter. It presents a story that intertwines the ingenuity of observers and experimenters, the creativity of theoreticians, exotic concepts such as MACHOs, WIMPs, quintessence, phantom energy and the modern-day reincarnation of the cosmological constant - a quantity that Albert Einstein injected into the cosmic mix nearly nine decades ago and subsequently derided as his `greatest blunder'. It leads the reader to the very frontiers of the quest to discover the nature, composition and ultimate fate of the universe.