Where is Dark Matter Hiding?

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Dark matter is one of those things that keep astronomers in business. It makes up 23 percent of the energy of the universe. However, we don't yet know what it IS. Particle physicists are on the hunt and a tantalizing possible detection was announced last December. New data from a different instrument, Xenon100, casts a rather big shadow of doubt.

Although a few astronomers still doubt the actual existence of dark matter, most of the evidence points to an as-yet-undiscovered particle that does not interact with "normal" matter or light in any obvious way. In fact, these WIMPs (Weakly Interacting Massive Particles) do not interact with themselves in the same way that "normal" matter does. This is nicely seen in the "Bullet Cluster" in which the dark matter is separate from the hot cluster gas (above). This result really drove home the concept for me, personally.

Last December, the Cryogenic Dark Matter Search II (CDMSII) announced that two dark matter particle events had been detected by their germanium and silicon detectors deep underground in a Minnesota mine. Though it was more likely that these events were due to cosmic rays or radioactivity, the results were consistent with a detection of WIMPs. The team did not, however, formally claim detection of dark matter, just a tantalizing possibility.

The Xenon Dark Matter Project's new Xenon100 is claimed to be more sensitive than CDMSII with 100kg of liquid xenon at an underground site in Italy. Xenon is a very heavy element, and thus has a larger cross section for collision with a WIMP. A new data set from this instrument shows no events, and thus suggests that dark matter was not detected by CDMSII, or yet another project in Italy, DAMA, which claims dark matter detection with seasonal variation as Earth moves around the sun.

However, each project has a different search strategy and instrumentation. It may not be possible, in the light of the fact that no one knows exactly how WIMPs will behave in reality, to directly compare these experiments. We may look back years later and realize that DAMA and CDMSII did make true detections, but at the present moment, there is not enough evidence to definitively say we have WIMPs. Maybe they will be created in the Large Hadron Collider soon? 

Image: These two galaxy clusters have slammed into and through each other. Most of the normal (baryonic) mass is in the hot gas detected by the Chandra X-Ray Observatory, shown in pink. The dark matter makes up most of the total mass, and dominates the gravitational lensing of the cluster, or the bending of light from background galaxies due to gravity. That distribution is shown in blue. Credit: X-ray: NASA/CXC/CfA/

M.Markevitch et al.;

Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/

D.Clowe et al.;

Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al