This encounter with the invisible began eight years ago, when two competing teams of astronomers were using exploding stars known as Type 1a supernovas as cosmic distance markers to determine the fate of the universe.

Ever since the Big Bang 14 billion years ago, the galaxies and the rest of the universe have been flying apart like a handful of pebbles tossed in the air. Astronomers reasoned that gravity would be slowing the expansion, and the teams were trying to find out by how much and, thus, determine whether all would collapse one day into a “big crunch” or expand forever.

Instead, to their surprise, the two teams, one led by Saul Perlmutter of the University of California, Berkeley, and the other by Brian Schmidt of the Mount Stromlo and Siding Spring Observatories in Australia, found that the universe was speeding up instead of slowing down.

But the ground-based telescopes that the two teams used could track supernovas to distances of just seven billion light-years, corresponding to half the age of the universe, and the effect could have been mimicked by dust or a slight change in the nature of the supernova explosions.

Since then, Dr. Riess, who was a member of Dr. Schmidt’s team, and his colleagues have used the Hubble telescope to prospect for supernovas and dark energy farther out in space or back in time.

The new results are based on observations of 23 supernovas that are more than eight billion years in the past, before dark energy came to dominate the cosmos. The spectra of those distant supernovas, Dr. Riess reported, appear to be identical to those closer and more recent examples. By combining the supernova results with data from other experiments like the NASA Wilkinson Microwave Anisotropy Probe, Dr. Riess and his colleagues could begin to address the evolution of dark energy.

“That’s one of the $64,000 questions,” he said. “Is dark energy changing?”

So far, he said, the results are consistent with the cosmological constant, but other answers are also possible. The possibility that it is the cosmological constant is a mixed blessing. Physicists concede that they do not understand it.