One of the world's leading astronomers tells the story of unlocking an astonishing cosmic secret. Supernova expert Robert Kirshner brings readers inside a lively research team on the quest that led them to an extraordinary cosmological discovery: the expansion of the universe is accelerating under the influence of a dark energy that makes space itself expand.Measurements of light from exploding stars--some of them halfway across the universe--let these astronomers trace the history of cosmic expansion. The results have been amazing. Instead of a universe slowing down due to gravity as theory predicted, observations reveal a universe whose expansion is speeding up. This measurement of dark energy--a quality of space itself that causes cosmic acceleration--points to a gaping hole in our understanding of fundamental physics.In 1917, Einstein proposed the \"cosmological constant\" to explain a static universe. When observations proved that the universe was expanding, he cast this early form of dark energy aside. But recent observations described first-hand in this book show that the cosmological constant--or something just like it--dominates the universe's mass and energy budget and determines its fate and shape.Warned by Einstein's blunder, and contradicted by the initial results of a competing research team, Kirshner and his colleagues were reluctant to accept their own result. But, convinced by evidence built on their hard-earned understanding of exploding stars, they announced their conclusion that the universe is accelerating in February 1998. Other lines of inquiry and parallel supernova research now support a new synthesis of a cosmos dominated by dark energy but also containing several forms of dark matter. We live in an extravagant universe with a surprising number of essential ingredients: the real universe we measure is not the simplest one we could imagine.This book invites any reader to share in the excitement of a remarkable adventure of discovery.