Craig Hogan, head of the Center for Particle Astrophysics, wrote this week's column.
When astronomers measure the motions of stars, gas and galaxies in our universe, they find that the motions require forces that go beyond the gravitational forces caused by ordinary, visible matter. Evidence for gravity from dark, unseen matter has piled up since scientists began these measurements in the 1930s.
For a long time scientists thought that dark matter might be swarms of invisible planets or gas in space. But substantial evidence now suggests that dark matter is something truly extraordinary. The favorite dark-matter candidates are whimsically called weakly interacting massive particles, or WIMPs, and Fermilab's Center for Particle Astrophysics is one of the best "Wimpstitutes" in the world. Our scientists are pursuing three different technologies to search for WIMPs: cryogenic crystals (CDMS), bubble chambers (COUPP) and liquid argon (Darkside).
The proposed existence of WIMPs fits well into our ideas for how the universe formed after the big bang. The hot soup of energy in the early universe apparently created both atoms and an even larger detritus of invisible particles that do not interact with light.
To explain the evolution and the matter density of our universe, dark-matter particles must be weakly interacting, such as neutrinos, but weigh much more than a neutrino: a single dark-matter particle must weigh as much as a heavy atom or large molecule.
We can look for WIMPS in the laboratory. To do that, scientists build ultra-sensitive particle detectors in laboratories deep underground, shielded from cosmic rays. There they look for rare, faint signals of light, sound or charge, caused by a WIMP bouncing off an ordinary atom in the detector. These WIMP interactions are rare. Even after a few years of taking data, there may only be one or two such events.
The CDMS scientists announced the observation of two candidate events last December. But they are cautious: these particle events could have been caused by still-unknown backgrounds.
Over the next few years, scientists will build better, larger detectors and deploy them underground in laboratories such as Soudan, SNOLab and DUSEL. The race is on: Who will be the first to discover what dark matter is made of? Stay tuned!