Dark matter may account for more than 90 percent of the matter in the universe. Yet its presence is so flimsy and whimsical that no one has ever been able to see it-not yet that is.

The WIMP Factor

Recent advances in detection equipment may bring the 70-year-old mystery of dark matter to a successful conclusion. A group of scientists participating in the Dark Matter experiment (DAMA) at Gran Sasso National Laboratories in Italy, with help from colleagues at the Chinese Academy of Sciences' Institute of High Energy Physics, claims that their experiments over the past 4 years have revealed the presence of 'weakly interacting massive particles' or WIMPs, which may contribute the largest share of energy to the universe. The research group is led by Rita Bernabei and Pierluigi Belli, who are affiliated with the University of Rome Tor Vergata and the National Institute of Nuclear Physics (INFN) in Italy.

No one should underestimate the importance of WIMPs' shadowy presence. Scientists believe that the gravitational pull they supply keeps the Milky Way from spinning out of control and flying apart.

Physicists have long suspected the presence of dark matter and there's plenty of proof-both theoretical and experimental--to back them up. Particles with verifiable mass--protons or neutrinos, for example--are simply not 'bulky' enough to create a force of gravity of sufficient strength to hold stars and other heavenly objects in place as they whirl around our universe at warp speed. For most physicists, the issue is not whether dark matter exists, but what is the nature of these elusive particles.

At times, researchers have pointed to such newly discovered particles as neutrinos or to massive stars labelled brown dwarfs. But subsequent theories and experiments have shown that neither these particles nor these stars have what it takes to fill the void: that is, they do not possess the physical features or act in ways that can account for the galaxies and clusters of celestial bodies that we observe in the sky. So the world's particle hunters have turned to WIMPs. These 'dark' particles, which have defied detection thus far, are now close to being cornered.

The conclusions of the DAMA researchers were first announced in Rome in January 1999 and subsequently presented to a group of physicists at the Fourth International Symposium on Sources and Detection of Dark Matter in the Universe held in Marina del Rey, California, in the United States, in February 2000.

Meanwhile, particle hunters from the Cold Dark Matter Search (CDMS) group at Stanford University in the United States, relying on another detection technique, suggested that their Italian counterparts may have detected not WIMPs but neutrons--a more ordinary particle that also emits flashes of light when subject to highly sophisticated laboratory conditions. Neutrons have already been ruled out as the constituent ingredient in dark matter; so, in the U.S. researchers' view, the hunt must continue.

The debate over dark matter, regardless of the outcome, holds the potential to both confirm and expand the research efforts of theoretical physicists at ICTP, the University of Trieste, the International School for Advanced Studies (SISSA) and elsewhere. After all, theoretical discussions during the past half century have provided the insights and inspiration for examining the deepest reaches of our universe and enhancing our understanding of the elementary constituents and forces that gave rise to the universe's birth and its current state of physical being.

As advances in technology place us on the threshold of discovering the makeup of dark matter, this historic finding is likely to raise as many questions as it solves. As one researcher recently noted, "the Copernican revolution told us we're not the center of the universe. The (imminent) discovery of dark matter will tell us we're not the main matter of the universe." All of which makes the 'WIMP factor' that much more intriguing.

Antonio Masiero
Astrophysics Sector
International School for Advanced Studies (SISSA)

For more detailed information about the scientific debate on WIMPs, see G. Jungman, M. Kamionkowski and K. Griest, "Supersymmetric Dark Matter," Physics Reports 267 (1996), pp. 195-373, and A. Masiero, "Dark Matter in the Universe," in S. Bonometto, J.R. Primack and A. Provenzale, eds., Proceedings of the International School of Physics "E. Fermi" Course CXXXII (Varenna, Italy: IOS Press, 1996).

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