If standard gravitational theory is correct, then most of the matter in the universe is in an unidentified form which does not emit enough light to have been detected by current instrumentation. This review volume is the extended editon of the lectures given in the 4th Jerusalem Winter School of Theoretical Physics, with new materials added in. It devoted to the discussion of the so-called "missing matter" problem in the universe - dark matter and dark energy. The goal of this volume is to make current research work on unseen matter accessible to students of faculties without prior experience in this area. Due to the pedagogical nature of the notes and the intense discussions between students and the lecturers, the written lectures included in this volume often contain techniques and explanations not found in more formal journal publications.