Jennifer Oullette in New Scientist:
Dark matter has just suffered another blow. Only one experiment claims to have seen signs of the mysterious stuff, and now the massive XENON100 experiment has failed to find any evidence for that signal. This may put the controversial signal to rest once and for all – but some say it’s not that simple.
Dark matter is a mysterious substance that makes up roughly 23 per cent of our universe. We know it’s there because of the gravitational force it exerts on normal matter, but it’s devilishly difficult to detect.
Myriad experiments have been trying to do just that, most buried deep underground to block out troublesome cosmic rays. But while there have been a few tantalising hints here and there, nothing has reached the threshold required to count as detection – with one exception.
In 1998, scientists at the DAMA experiment buried deep in Italy’s Gran Sasso mountain claimed to have detected dark matter in the form of a weakly interacting massive particle (WIMP) weighing around 10 gigaelectronvolts (GeV). The rate of recorded blips as particles collide with the nuclei of the detector material varied with the seasons. The DAMA scientists attributed this to the Earth moving through a “wind” of dark matter as it orbits the sun.
DAMA’s signal was unmistakable, but many physicists argued that other factors besides dark matter could explain it. It didn’t help that the DAMA team refused to share their data publicly or collaborate with other researchers, making it more difficult to test those claims.