It is 15 head-scratching years since we noticed that some mysterious agent is pushing the universe apart. We still don't know what it is. It is everywhere and we can't see it. It makes up more than two-thirds of the universe, but we have no idea where it comes from or what it is made of. "Nature has not been ready to give us any clues yet," says Sean Carroll, a theoretical physicist at the California Institute of Technology in Pasadena.
We do at least have a name for this most enigmatic of beasts: dark energy. Now the hunt for it is really on. Later this year, astronomers will begin a new sky survey to look for signs of the stuff among exploding stars and ancient galaxy clusters. A pack of space missions and gigantic Earth-based telescopes will soon join the chase. Meanwhile, some physicists are pursuing an unorthodox idea: that we might snare dark energy in the lab.
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