Around 85 percent of the matter in our universe exists in a form that we cannot directly observe and that interacts only very weakly with the atoms of matter known to us. Dark matter is a huge, perhaps the biggest mystery of cosmology. Those trying to understand it brush upon almost all of the topics that our cosmos has to offer: from the development and dynamics of galaxies to the origins of our universe as a whole. This volume provides an overview of potential candidates for this strange form of matter and discusses the current issue of whether there are alternatives to the dark matter hypothesis.