This could have been the day they discovered dark matter.

On the morning of April 4, a dozen or so graduate students and postdoctoral fellows gathered in the offices of Elena Aprile, a physics professor at Columbia University, to get their first look at the data from an experiment on the other side of the world. In a tunnel deep under Gran Sasso, Italy, Dr. Aprile and an international team of scientists had wired a vat containing 134 pounds of liquid xenon to record the pit-pat of invisible particles, the so-called dark matter that astronomers say constitutes a quarter of the universe.

Photographers were on hand to record the action — after all, you never know — although theoretical calculations suggested that with only 100 days of observation, the xenon experiment was probably still shy of the time necessary to see dark matter. “We will not discover dark matter today,” Dr. Aprile said. “We will be doing this again and again.”

Dark matter has teased and tantalized physicists since the 1970s when it was demonstrated that some invisible material must be providing the gravitational glue to hold galaxies together. Knowing what it is would provide a roadmap to new particles and forces, a new view of what happened in the Big Bang, and more Nobel Prizes than you can count. Failure to find it would mean that Einstein did not get the laws of gravity quite right.

The best guess is that this dark matter consists of clouds of exotic subatomic particles left over from the Big Bang and known generically as wimps, for weakly interacting massive particles, which can pass through the Earth like smoke through a screen door.