In Lykken's words, and as borne out by discussions at the meeting here on Friday, "theorists don't have a clue" as to the identity of this dark energy that is so important.

One leading candidate is something invented by Albert Einstein called the cosmological constant. But physicists despair of understanding Einstein's constant. Efforts to calculate it wind up missing by a factor of 10 to the 60th power, which has been called the worst mismatch in the history of science. Another candidate that has won some following among theorists lately is that Einstein was wrong: The acceleration is sort of an illusion that can be dispelled by modifying his general theory of relativity, the sourcebook of cosmic gravity.

The stakes for the universe are not small. If the cosmological constant is the culprit, the universe will expand faster and faster as time goes on. "It will be cold and dark in a few billion years," said Dr. Frank Wilczek, a physicist at the Massachusetts Institute of Technology, adding, "That would be very sad."