More than one thousand scientists from around the world have come together in the Legacy Survey of Space and Time (LSST) Dark Energy Science Collaboration (DESC) to explore the secrets of the dark Universe. Many questions demand answers: What is the cause of the late-time accelerated expansion of the Universe? Is dark energy a static vacuum energy (a cosmological constant), or does it have a dynamical component? What is its origin and how does it fit into the Standard Model of particle physics? What does the comparison between the expansion history and the growth rate of large-scale structure tell us about possible modification of General Relativity on the largest scales? What is the nature of dark matter? What happened during the very first moments of the Universe? What is the neutrino mass scale? DESC is hoping to answer many of these questions with the unprecedented dataset to be collected by the Vera C. Rubin Observatory (Rubin).