Where did our universe come from? People have been trying to answer this question for thousands of years. The twentieth century brough new discoveries in physics and astronomy that led scientists to develop the Big Bang theory—a detailed idea that describes how our universe formed. According to this theory, the entire universe began in a single instant, in an unimaginably powerful explosion. That explosion created all time and space, all matter and energy—everything in the universe as we know it. This book tells the story of how scientists' observations of the stars led to the development of the Big Bang Theory.