Now, an international team of astronomers led by scientists at the University of Manchester, have conducted a 10-year census of the sky for examples of gravitational lenses. This phenomena is observed when a galaxy bends the light from a distant bright galaxy called a quasar to form several images of the same quasar.

By linking the number of lenses discovered with the latest information on the numbers of galaxies, the scientists have posited that most of the energy in the Universe is likely to be in an invisible, and presently unknown, form.

Dark Energy only affects the properties of the Universe over very large Distances, astronomers say. As a result, the observations sensitive to its presence, in particular studies of exploding stars in distant galaxies, are all close to the limit of current capabilities.

"The new gravitational lens test is based on completely different physical arguments to the previous ones and so provides independent evidence in support of dark energy," said Ian Browne, one of the study team members.

When a quasar is gravitationally lensed by an intervening galaxy two or more images of the quasar are produced, but they are hard to recognize as the images are extremely close together. The team therefore employed several of the world's most powerful radio telescope arrays to make radio pictures of thousands of distant quasars.

"We chose to use radio telescopes for our survey since they can pick out details many times finer than optical ones, even the Hubble Space Telescope," said Peter Wilkinson, also on the team.

The census showed that about one out of every 700 distant quasars is lensed by a foreground galaxy.

To calculate the fraction of the energy in the universe that is dark energy, Manchester's Kyu-Hyun Chae combined the gravitational lens statistics with the latest results on the numbers and types of galaxies in the universe made with optical telescopes. The result suggest that around two-thirds of the universe's energy is dark energy.

The remaining third is made up of dark matter, whose form is presently unknown, and "ordinary" matter which makes up the stars and planets. For both of these forms of matter gravity acts as normal and attracts.

In contrast dark energy has long-range anti-gravity properties and now appears to be causing the expansion of the universe to accelerate, rather than slow down as would be expected if gravity was the dominant force. While astronomers have no idea about what dark energy might be, these new results add to their growing confidence that it is real.

The Lowdown on Dark Energy