AMUNDSEN-SCOTT STATION, ANTARCTICA -- Anywhere on Earth this would be a big telescope, as tall as a seven-story building, with a main mirror measuring 32 1/2 feet across. But here at the South Pole, it seems especially large, looming over a barren plain of ice that gets colder than anywhere else on the planet.

Scientists built the instrument at the end of the world so they can search for clues that might identify the most powerful, plentiful but elusive substance in the universe: dark energy.

Though dark energy is believed to account for 70% of the universe's mass, it is invisible and virtually undetectable. Nobody knows what it is, where it is or how it behaves.