An international team of astronomers, led by scientists at the University of Manchester have produced new evidence that most of the energy in the Universe is in the form of the mysterious "dark energy". The new evidence comes from a 10-year census of the sky for examples of gravitational lenses, which are seen when a galaxy bends the light from a distant quasar to form several images of the same quasar. Linking the number of lenses they found with the latest information on the numbers of galaxies, the scientists have been able to infer that most of the energy in the Universe is likely to be in an invisible, and presently unknown, form.

Dark energy is closely related to the idea of a Cosmological Constant introduced by Einstein over 80 years ago, but most astronomers, including Einstein himself, have always strongly doubted its reality. However, in the past 5 years several independent groups of astronomers have amassed evidence suggesting that dark energy exists and could well dominate the total energy of the Universe.

Dark energy only affects the properties of the Universe over very large distances. As a result, the observations which are sensitive to its presence, in particular studies of exploding stars in distant galaxies, are all close to the limit of current capabilities. Astronomers have therefore been keen to exploit many different tests and Dr. Ian Browne makes the point that "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".

When a quasar is gravitationally lensed by an intervening galaxy two or more images of the quasar are produced but they are hard to recognise as the images are less than one thousandth of a degree apart. The team therefore employed several of the world's most powerful radio telescope arrays to make radio pictures of thousands of distant quasars. Professor Peter Wilkinson points out that "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". 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 which is dark energy Manchester's Dr. 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 which emerged is that around two thirds of the Universe's energy appears to be 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.