Dark matter was discovered 80 years ago when astronomer Fritz Zwicky spied a galaxy cluster whirling so fast, the galaxies were bound to fly apart unless something — something less luminous than ordinary stars or gas — held them together.
Decades later, the scientific community concedes the existence of dark matter, after many different kinds of experiments and simulations, but physicists still don't know what it is.
But confidence is growing. The last 15 years have seen the construction of several exotic detectors buried deep underground, and those detectors may be giving us our first view of what dark matter is made of. In January's cover story, Dan Hooper discusses these experiments, their results, and their most recent find: a seasonal variation in detections that indicate Earth might be moving through a WIMP wind as it orbits the Sun.
We summarize the underground experiments below. All of these experiments are continually increasing their detector mass, so the masses listed here are not fixed.
|Guide to Dark Matter Experiments|
|Gran Sasso, Italy||250 kg sodium iodide crystals||1998||Thousands of events with annual variations|
|Soudan, Minnesota||CDMS-II: |
4.5 kg germanium crystals
9 kg germanium crystals
|2003||No claimed WIMP detections yet|
|CoGeNT||Soudan, Minnesota||500 g germanium crystal||2004||Hundreds of events with possible annual variations|
|CRESST||Gran Sasso, Italy||2.4 kg calcium-tungstate crystals||2006||Tens of events, possible WIMP signal|
|XENON-100||Gran Sasso, Italy||161 kg liquid xenon||2009||No claimed WIMP detections yet|
|LUX||Homestake, South Dakota||350 kg liquid xenon||2011||No science results yet|
|XMASS||Kamioka, Japan||800 kg liquid xenon||2011||No science results yet|