Astronomers announced new evidence yesterday that the mysterious force known as "dark energy" is causing the universe to expand ever more rapidly, perhaps eventually leading to cosmological "loneliness," in which galaxies grow so far apart that the heavens will appear empty.

The new observations, using NASA's orbiting Chandra X-ray Observatory to focus on distant galaxy clusters, supported earlier findings suggesting that dark energy is what Albert Einstein called the "cosmological constant," a gradual and predictable expansion of the universe that will result in total isolation in 100 billion years or so.

"This is clear and direct evidence that the expansion of our universe is accelerating," Cambridge University astronomer Steven Allen, leader of the Chandra study, said at a news conference. "Our results are consistent with the cosmological constant, and they have important implications for the fate of the universe."

Despite the new observations, Allen and others acknowledged that the strength of dark energy may not be constant. Other theories suggest that dark energy is weakening, sucking the universe toward an eventual cosmic collapse known as "the big crunch," or that it is growing stronger, accelerating the universe's expansion until it flies apart in a "big rip."

"More [observations] will shrink the error, but I think at the moment we are a relatively long way from knowing whether we are precisely at the cosmological constant," said astronomer Mario Livio of the Space Telescope Science Institute. "In some sense it may be impossible to know. If you deviate by one part in 100,000, it means you aren't close at all over a long period of time."

And cosmological doomsday, if it happens at all, will not occur for "tens of billions of years," said astrophysicist Michael S. Turner of the University of Chicago and the National Science Foundation.

For decades scientists assumed that gravity was gradually decelerating the universe's expansion after the "Big Bang" that formed the universe 13.7 billion years ago in an explosion of unimaginable dimensions.

Then, in 1998, researchers showed that about 6 billion years ago a mysterious "repulsive force" trumped gravity, re-accelerating the universe. This force has come to be known as dark energy, and subsequent research has indicated that it comprises 75 percent of the cosmos. "In a simple sense, it's the most important thing out there," said Chandra team member Andy Fabian, a Cambridge University astronomer.

And one of the least understood. "We can be confident the universe is speeding up," Turner said. "Why? That may be the most profound problem in all of science."

Earlier research supported the presence of dark energy by measuring the properties of the light emanating from exploding stars called supervnovas. Later studies of cosmic microwave background radiation also suggested dark energy.

The Chandra team used the orbiting X-ray telescope to observe 26 galaxy clusters 1 billion to 8 billion light-years away. The X-rays enable scientists to see the hot gases that make up the bulk of the conventional matter in the cluster.

The team then determined the ratio of hot gas to "dark matter," unseen material so named because scientists know so little about it. Finally they calculated the distance to the clusters. "The galaxies were all much further away than they should have been, " Allen said. The difference is because of dark energy.

University of Chicago theoretical physicist Sean Carroll welcomed the Chandra data, noting that independent verification of something that is still "a new idea" helps convince "people out there that still don't believe it."

But the data left open critical questions. "If the universe is accelerating, how did it evolve over time?" Carroll asked. "We have the favorite answer -- that it's expanding at a constant value. That fits the data, but it makes no sense to us. Why should it be that particular value? Why isn't it much bigger? Or much smaller?"

And despite growing evidence, the whole concept of dark energy remains "strange, quirky, bizarre -- and unacceptable," added Princeton University astrophysicist John Bahcall. "But there it is. It's one thing to say you're missing a few pennies in the jar, but it's fairly scandalous when we don't even know what most of the universe is composed of."