Computer simulation of the distribution of dark matter that is thought to fill the Universe. Credit: INST. THEOR. PHYS., UNIV. ZURICH

Physicists have again returned empty-handed from a search for the 'dark matter' that is thought to fill the cosmos. But the latest null result hasn't dimmed their enthusiasm — or their plans for a new generation of detectors.

Since the 1970s, theorists have predicted the existence of massive particles that rarely, if ever, interact with normal matter. This dark matter is believed to be responsible for slowing the rotation of galaxies and makes up about 85% of matter in the Universe. Physicists have devised a host of experiments to find dark matter, but to date, nobody has been able to detect it directly (see Nature 448, 240–248 ; 2007).

The latest non-findings met with spontaneous applause at a conference on 22 February in Marina del Rey, California. The results came from Cryogenic Dark Matter Search II (CDMSII) — one of the world's most advanced dark-matter detectors, located in the Soudan mine in Minnesota. CDMSII uses giant crystals of germanium and silicon that physicists hope will ring out when struck by a dark-matter particle.

The crystals are kept more than 700 metres underground in order to shield them from disruptive cosmic radiation. A previous generation of CDMS detectors delivered nothing, and the new bigger and better crystals are only now beginning to reach sensitivities at which they might realistically see a dark-matter particle, says Bernard Sadoulet, a physicist at the University of California, Berkeley, who leads the CDMSII collaboration. “We really are entering the core of the interesting territory.”

The results are not entirely surprising, says Elena Aprile, a physicist at Columbia University in New York city who is working on a rival experiment called XENON10. The dark-matter particles could still be well below the experiment's threshold, she says. “Maybe it's just a little too insensitive still.”

Sadoulet says the CDMSII detector should reach up to four times its present sensitivity later this year, and Aprile says a new, larger XENON100 detector could do better still. But detecting the particles may require sensitivities hundreds of times beyond even these levels. That will probably mean a next generation of detectors. Sadoulet and Aprile both say they have proposals for third-generation detectors, which are expected to cost tens of millions of dollars.

But the lack of detection and rising costs raise another spectre for the dark-matter hunters — what if the particles aren't there? Or what if they don't interact with regular matter? Nobody knows exactly where dark matter lives in the Milky Way, although it is likely to exist in our Solar System in some shape or form, according to Ben Moore, a theorist at the University of Zurich in Switzerland. But he adds that many proposed versions of the stuff would never interact with a germanium crystal — or anything else. “It's a gamble,” he says. “If dark matter is one of those alternative candidates, then they're not going to see anything.”

Sadoulet is the first to admit that this may be the case. Still, he hastens to add, there is good reason to look, especially because several other experiments may soon provide further clues. A satellite called the Gamma-ray Large Area Space Telescope will launch later this year, and may provide evidence of dark matter 'annihilating' in the cosmos. Also this year, the Large Hadron Collider, the world's largest particle accelerator, which is located at CERN near Geneva in Switzerland, will start looking for signals that could confirm the existence of some classes of dark-matter particle.

The risk of coming up empty-handed nags at the field, Sadoulet says. But if something is found, “it will be a new start for particle physics”.