Saul Perlmutter of the University of California - Berkeley, Adam Riess of Johns Hopkins University, and Brian Schmidt of Australian National University's Mount Stromlo and Siding Spring Observatories have been awarded the 2011 Nobel Prize in Physics for discovering that galaxies are receding from one another faster now than they were billions of years ago. The accelerating expansion means that the universe could expand forever until, in the distant future, it becomes cold and dark. The teams' discovery led to speculation that there is a "dark energy" that is pushing the universe apart. Though dark energy theoretically makes up 73 percent of the matter and energy of the universe, astronomers and physicists have so far failed to discover the nature of this strange, repulsive force.