The Nobel Prize in physics was awarded today to Brian Schmidt (Australian National University), Adam Riess (Johns Hopkins University), and Saul Perlmutter (University of California, Berkeley). According to this
Los Angeles Times article, the three earned the award for "discovering that the universe is moving faster and faster outward -- movement likely powered by the mysterious 'dark energy' that fills about 70% of the universe." You can find introductory essays on dark energy in the index in the right-hand column.