A journey into one of the greatest mysteries of modern science, this book explores the hidden world of dark matter - and the proposition that it may reside in a parallel "mirror" universe existing alongside our own. Astronomical observations have established that most of the matter in the universe is in an invisible form, known as dark matter. Following its discovery by Fritz Zwicky in 1930 and confirmation by Kent Ford and Vera Rubin in the 1970s, NASA's 2001 explorer mission provided further proof that dark matter constitutes more than 80% of matter in the universe. It has since become clear that dark matter explains how galaxies formed from the uniformly expanding cosmic fluid as the universe evolved, thereby solving another major mystery of the cosmos.