This book begins by reviewing the evidence for dark matter, particularly in our own Galaxy where measurements are most accurate. The candidates for various types of dark matter are investigated as well as the ongoing attempts to detect them. Those alternative theories to dark matter for solving the "missing mass" in the universe which cannot yet be ruled out are also briefly presented. The author`s stated aim of the book is to demonstrate why astronomers think dark matter exists and to describe the current state of investigations into its as yet undetermined nature.