Dark energy is a mysterious force causing the universe to expand in an accelerating manner. Although Edwin Hubble discovered observational proof of expansion in 1929, the first direct evidence for dark energy came from supernova observations in the late 1990s. Untangling complex physics theories with accessible language, informative sidebars, and captivating imagery, this text explores the development and evaluation of scientific theories behind dark energy. Students will learn about the tools and methods scientists use to unravel dark energy's origins, dynamics, and effects on the universe.