In 2006, two clusters of galaxies provided the first direct evidence of the existence of the universe's mysterious dark matter. Now, there's a tantalizing hint that this dark matter is made up of weakly interacting massive particles.

Credit: NASA

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SYDNEY: There is a 75% chance that scientists have detected dark matter, in the form of weakly interacting massive particles (WIMPs), at an experiment deep underground in Minnesota, USA.

One of the most popular candidates for dark matter - the elusive, invisible substance thought to make up around 85% of the mass of the universe - are weakly interacting massive particles, or WIMPs.

WIMPs are hypothesised dark matter particles that scientists predict could occasionally, although very rarely and weakly, interact with normal matter. When this occurs, the dark matter particles would scatter from an atomic nucleus like billiard balls, leaving behind a small amount of energy detectable under the right conditions.

Detecting WIMP interactions

The Cryogenic Dark Matter Search (CDMS-II) experiment has been searching for WIMPs in detectors located 800 m underground in the Soudan mine in northern Minnesota, USA, since 2003.

In the mine, 30 detectors made of germanium and silicon crystals are cooled to near absolute zero (-273 degrees Celsius) and equipped with special sensors to detect potential WIMP scattering events.

When a particle interacts in the cooled detectors, it releases energy in the form of heat and electric charge. The sensors are then able to detect and record the two signals for later comparison, enabling scientists to tell whether the particle was a WIMP or one of numerous other known background particles, such as solar and cosmic rays.

Two possible detections of dark matter

The results of the latest data set taken from the CDMS-II experiment over 2007 and 2008 and published in the online version of the US journal Science show the detection of two possible WIMP events.

However, in order to claim a discovery there must be less than one chance in a thousand that the detected signal is due to background events. In this case, that would have required the observation of five events.

With just two observed, however, it is estimated that there is a one in four chance that they may have instead resulted from 'background noise'.