Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Black holes are regions of spacetime where the gravitational field is so powerful, that it prevents anything, including light, from escaping. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that marks the point of no return. Learn more about black holes, some of the most massive and mysterious objects know to exist and their impact on the evolution of the universe in this book.