Why is the Universe expanding more and more rapidly? The Dark Energy Spectroscopic Instrument (DESI) is being designed to try to find out why. Is the acceleration due to a cosmological constant? a dynamical dark energy field? a failure of General Relativity? New instrumentation of the 4-meter telescope Mayall telescope will enable us to measure more than 20 million galaxy redshifts and find the imprint of waves that propagated in the cosmic plasma for the first 380,000 years. That imprint will map the expansion history of the Universe with unprecedented accuracy and may answer the biggest question in physical science.