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 proceedings was devoted to a discussion of the so-called "missing matter" problem in the universe. The goal of the School was 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 School and the strong interactions between students and the lectures, the written lectures included in this volume often contain techniques and explanations not found in more formal journal publications.