By Dr David
BBC News Online science
A US team has released the first results from a super-sensitive hunt for the mysterious "dark matter".
The heart of the dark matter detector
This form of matter comprises more than 70% of the Universe's mass, far more than the stars and galaxies we can see.
The Cryogenic Dark Matter Search uses equipment at the bottom of a Minnesota mine to filter out all interference.
Writing in the Physical Review Letters, the team says that while a detection has yet to occur, there is now a better idea of how much dark matter must exist
The underground observatory is some 1,000m beneath the surface. The Earth's crust shields its germanium detectors from cosmic rays and the background particles they produce.
It is only from such an isolated place that scientists believe they have a chance of catching their quarry.
Dark matter pervades the Universe - its presence is betrayed by its gravitational influence on stars and galaxies, but astronomers do not know what it is.
The scientists hope that if their detectors are sensitive enough and they wait long enough, a dark matter particle will finally be spotted.
A favoured theory is that the dark matter consists of Wimps (weakly interacting massive particles) about a thousand times more massive than a proton, one of the particles found in an atom's nucleus.
This means that on the rare occasions a Wimp strikes an ordinary atom, the effect should be noticeable.
The CDMS 2 (Cryogenic Dark Matter Search) result is the most sensitive search for Wimps yet carried out.
The equipment is placed deep underground
There has been no Eureka moment, but the scientists involved say the experiment has now given them a better upper limit on the abundance of Wimps.
They report the interaction rate of Wimps with their detectors is less than one interaction every 25 days per kilogram of germanium - a measurement four times more sensitive than previously obtained.
"Over the next few years, we hope to improve our sensitivity by a further factor of 20 or more," said Dr Blas Cabrera, of Stanford University, US.