The expansion of the universe can be tracked by measuring changes in supernovae, such as this before-and-after picture taken by NASA's Hubble space telescope. ((NASA))

Scientists working with NASA's Hubble space telescope are getting a clearer picture of dark energy, the mysterious force behind the expansion of the universe.

"Our latest clue is that the stuff we call dark energy was relatively weak, but starting to make its presence felt nine billion years ago," researcher Adam Riess said on Thursday.

"Although dark energy accounts for more than 70 per cent of the energy of the universe, we know very little about it, so each clue is precious," he said in a NASA release.

The finding that dark energy has existed for most of the universe's history will help scientists by ruling out a competing explanation that held that the force changed over time.

Astrophysicists are particularly interested in two of dark energy's fundamental properties: its strength and its permanence.

The new observations show it was present and countering the force of gravity long before dark energy began to force the universe apart.

Previous Hubble observations show that gravity was slowing theexpansion of the universe. But the expansion rate began to speed up about five to six billion years ago, when dark energy's repulsive force overcame gravity's attraction, astronomers believe.

Hubble studied dark energy by looking at the behavior of the 24 most distant supernovae, most found in the past two years. The supernovae, stars that exploded eons ago, are too far away to be studied by ground-based telescopes.

The change in supernovae can be used to trace the universe's expansion, NASA said, likening them to the marks parents make on a door frame to measure a child's growth.

The Hubble observations are consistent with a prediction made nearly a century ago by Albert Einstein. He thought there must be energy coming from empty space forcing the universe apart, or else gravity would cause the universe to collapse on itself.

Einstein's "cosmological constant" was only a hypothesis until 1998, when Riess and others figured out that space was expanding at an increasing rate, based on supernovae observations.