The data comes from a three-year study of exploding stars

That is the conclusion of astronomers who presented results from a three-year study using the Hubble Space Telescope.

The finding may rule out some competing theories that predict the strength of dark energy changes over time.

Dark energy makes up about 74% of the Universe; the rest is dark matter (22%) and normal matter (4%).

Understanding the nature of dark energy is arguably the biggest problem physics is facing today Mario Livio, Space Telescope Science Institute

"It appears this dark energy was already boosting the expansion of the Universe as much as nine billion years ago," said co-investigator Adam Riess from the Space Telescope Science Institute in Baltimore, US.

"That's out of a Universe which we think is about 13.7 billion years old - most of the way back."

The findings are consistent with the idea of dark energy behaving like Albert Einstein's cosmological constant. The cosmological constant describes the idea that there is a density and pressure associated with "empty" space.

In this scenario, dark energy never changes; it has the same properties across the age of the Universe.

Repulsive force

Einstein first conceived of the notion of a repulsive force in space in his attempt to balance the Universe against the inward pull of its own gravity, which he thought would ultimately cause the Universe to implode.

His cosmological constant remained a curious hypothesis until 1998, when astronomers used observations of supernovae from ground-based telescopes and Hubble to show that the expansion of space was accelerating.

These findings suggested there really was a repulsive form of gravity in space, a force that was shortly dubbed "dark energy".

There have been many attempts to explain the nature of dark energy.

One of these is that it behaves like the cosmological constant. Another is that dark energy behaves like a field that changes over time. The third proposes changes to our theories of gravity to explain the mysterious force.

The latest data from Hubble contradict theories that dark energy might have behaved differently billions of years ago to how it behaves now, or might not even have been present. Some astronomers had thought that dark energy might mimic whatever was the dominant force in the Universe at the time, such as matter for example.

Previous Hubble observations of the most distant supernovae known revealed that the early Universe was dominated by matter whose gravity was slowing down the Universe's expansion rate.

The observations also confirmed that the expansion rate of the cosmos began speeding up about five to six billion years ago. That is when astronomers believe that dark energy's repulsive force took over from that of gravity.

'Tug of war'

"Imagine that you were having a tug of war and the other end of the rope disappears behind a curtain. Somebody else is tugging on the other end; we'll call that dark energy," said Dr Riess.

"In 1998, we saw that the thing behind the curtain was winning, it was pulling harder and the Universe was accelerating.

"In 2004, we showed that was not always the case. There was a time when you - ordinary matter - were winning. The Universe was decelerating. Now, we have shown that, even at that time, the thing on the other end of the rope was beginning to pull."

The discovery comes from observations of 23 exploding stars, or supernovae. Using Hubble to peer far across the Universe, the astronomers were able see back to a time when the cosmos was less than half its present size.

"These supernovae provide cosmic mile-markers that allow us to measure the growth rate of the Universe about nine billion years ago," said Adam Riess.

Mario Livio, of the Space Telescope Science Institute, added: "Understanding the nature of dark energy is arguably the biggest problem physics is facing today."