The Nobel Prize for physics in 2011 was awarded to Brian Schmidt, Adam Riess, and Saul Perlmutter for discovering that the universe is expanding at an accelerating rate. This finding was completely unexpected because it was thought that gravity should slow the expansion of the cosmos. The best current explanation of why the universe is accelerating is that there is some energy tied to empty space which pushes matter apart. This 'Dark Energy' makes up 73% of the universe but is very difficult to detect. Images courtesy of NASA/NASAimages.org and Maritza A. Lara-Lopez