One of the great mysteries in physics why 90 percent of the universe's mass appears to be invisible came one step closer to resolution last week.

A new sensor system that can pinpoint the presence of a theorized form of "dark matter" known as "WIMPs" (Weakly Interacting Massive Particles) has released its first findings though the results seem to raise as many questions as they answer.

The new detector was developed by the Cryogenic Dark Matter Search (CDMS), a collaboration of 10 institutions and researchers from around the world. Its advantage is that it can discriminate between "hits" caused by WIMP impacts and other phenomena that cause similar impact effects.

At the heart of the CDMS sensor is a mass of silicon and germanium crystals. The CDMS detector works by sensing the recoil energy when a particle strikes the nucleus of an atom in these crystals, each impact being sensed as ionization and heat. Heat is the result of atoms vibrating, but normally the tiny amount of heat caused by a single tiny impact would make such a small difference in the temperature of the sensor that it would not be detectable.

But that is where the cryogenic feature of the detector comes in: By cooling the crystals almost to absolute zero, the normal atomic vibrations in the material of the sensor are almost completely stopped. With the normal heat vibration virtually stopped, any vibrations caused by a particle impact become much more evident and they can be counted as "hits."

However, the CDMS sensor can not only count hits, it can also tell what kind of "hit" it was. By carefully measuring the amount and type of energy released by each impact, it can tell if the impact was that of common radiation (such as beta and gamma rays) or of a heavy particle such as a WIMP. The sensor does this in two ways:

• It measures the level of ionization caused by each impact. Beta and gamma ray impacts cause relatively high levels of ionization, and thus any such impacts can be eliminated as not being WIMP impacts.
• In addition, the detector can measure the amount of heat energy released by the impact and therefore tell if it was a heavy-particle impact on an atomic nucleus, which could be due to a WIMP. It's analogous to listening to bullets striking a heavy metal plate: A small-caliber bullet would barely affect the metal plate, while a big bullet would set it ringing like a gong. The CDMS detector can measure the amount of "ringing" caused by each impact (measured as heat from the detector) and thereby tell if the impact was caused by a light impact (such as from a gamma ray) or from a heavy particle such as a WIMP.

And yet, to date the main finding of the detector has been nothing. In its first year of operation the CDMS detector has recorded 13 nuclear impacts -- most or all of which could be explained by things other than WIMPs. However, this finding is itself quite significant because it is at odds with results from an earlier type of WIMP detector that seemed to yield many more hits.

The large discrepancy may seem strange, but even stranger is that both results may be correct. According to Dr. Bernard Sadoulet, one of the researchers involved in the project, "[The] physics for this unknown particle may be different from what we expect, and it is theoretically possible that we are both right!"

The quest to verify WIMP theory recently became more urgent because of the partial demise of its competitor, which is known as MACHO theory. The theory of MACHOs (MAssive Compact Halo Objects) holds that the "missing matter" is not made up of elusive subatomic particles, but by galaxies having a "halo" of plain old everyday matter that happens to be dark and therefore invisible. This would include burned-out dark stars, stray planets and other large, heavy, but dark clumps of common everyday matter. Such objects would be invisible to telescopes, but if they were ubiquitous throughout the universe they could account for the "missing matter."

To confirm the existence of MACHOs, telescopes have been set up to watch for the brief "eclipses" caused by these dark objects moving in front of distant stars. But after more than five years of searching, very few potential MACHOs have turned up, and MACHO researchers recently reported that they now believe that MACHOs cannot account for most of the "missing matter."

The elimination of MACHOs as the primary explanation of the "missing matter" is providing additional impetus for improved WIMP theories and WIMP detectors. Plans are already underway for larger WIMP detectors, while theoreticians race to find new theories that would explain the conflicting results, as well as help devise improved detectors. The potential end result confirmation of the existence of a whole new type of matter that makes up most of the universe would have enormous implications for our understanding of the universe, and thus this is expected to remain a hot research area for years to come.