Jeremiah Ostriker received his doctorate in astrophysical sciences from the University of Chicago, under the tutelage of the legendary astrophysicist and Nobel laureate, S. Chandrasekhar. After receiving his doctorate, Jerry held a postdoctoral position at Cambridge University. He then went to Princeton University, where he became the Chair of the Department of Astrophysical Sciences and the Director of Princeton University Observatory. Since 1995 he has served as the Provost of Princeton University, while maintaining his position as a professor in the Department of Astrophysical Sciences. Jerry Ostriker's contributions to astrophysics have earned him the recognition of his colleagues in awards as diverse as the Helen B. Warner Prize and the Henry Norris Russell Prize of the American Astronomical Society, the Vainu Bappu Memorial Award of the Indian National Science Academy, and the Karl Schwarzschild Medal of the Astronomische Gesellschaft of Germany. Jerry's research spans much of the field of theoretical astrophysics, with his current interests focusing on cosmology. For this book, he chose to write about one of astronomy's longest-standing mysteries: the dark matter that pervades galaxies.