Scientists now have evidence that dark energy has been around for most of the universe's history.

Using NASA's Hubble Space Telescope, researchers measured the expansion of the universe 9 billion years ago based on 24 of the most distant supernovas ever known.

As theoretically expected, they found that the mysterious antigravity force, apparently pushing galaxies outward at an accelerating pace, was acting on the ancient universe much like the present.

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All supernovas of a certain variety, called Type-1a, burn with the same brightness, so scientists can calculate relative distances in the universe based on how dim or bright these exploding stars get.

In the late 1990's it was realized that these standard candles were dimmer than expected and that the expansion of the universe was accelerating.

Scientists blamed the acceleration on an inexplicable repulsive force — dark energy.

"Although dark energy accounts for more than 70 percent of the energy of the universe, we know very little about it, so each clue is precious," said Adam Riess, a professor at Johns Hopkins University who was involved in the initial discoveries back in the '90s. "Our latest clue is that the stuff we call dark energy was present as long as 9 billion years ago, when it was starting to make its presence felt."

The universe is about 13.7 billion years old.

Previous observations revealed that the early universe was comprised of matter whose gravity was trying to pull it all inward and slow down its expansion. But the spreading out of the cosmos started speeding up around 5 billion to 6 billion years ago. That's when scientists believe dark energy started to win the cosmic tug of war.

"After we subtract the gravity from the known matter in the universe, we can see the dark energy pushing to get out," said Lou Strolger from the University of Western Kentucky.

The findings were announced in a media teleconference at NASA headquarters today and will be published in the Feb. 10 issue of Astrophysical Journal.

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