Dark energy may not exist, according to an international team of astronomers who have used an X-ray satellite to count galaxy clusters in the early universe. If they are right, the expansion of the universe is not speeding up and the scientific community has been taken in by a huge cosmic mirage. The idea of dark energy was first suggested in 1998 when a team led by Saul Perlmutter at the University of California at Berkeley reported that distant supernovae appeared fainter than they should, given their distance from the Earth. This could be explained by the expansion of the universe speeding up in the time the supernovae's light had taken to reach us, driving them farther away than expected and making them appear fainter. Empty space appeared to be filled with dark energy, a weird anti-gravity stuff that was remorselessly driving the galaxies apart. More recent observations of the dim "afterglow" of the big bang fireball, made with NASA's Wilkinson Microwave Anisotropy Probe (WMAP) and announced earlier this year, appeared to confirm the existence of dark energy and so the astronomical community settled on a universe that was 30 per cent matter and 70 per cent dark energy. It is this conclusion that is now under attack.
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