Ups and Downs in the Search for Dark Matter

We know dark matter is out there. Astrophysical observations of gravitational effects provide evidence of its existence at all times, from the time of nucleosynthesis (a few minutes after the big bang) to the moment in which the cosmic microwave background radiation was released (300,000 years after the big bang), all the way to the present Universe. Dark matter pervades all length scales, from the Universe as a whole to individual galaxies, including our Milky Way, and even to some smaller structures. The question is not, does dark matter exist? The question is, what is dark matter made of?

In separate reports, one appearing in Physical Review Letters [1], the other on the arXiv [2], two collaborations—the Cryogenic Dark Matter Search (CDMS) experiment at Fermilab in Illinois and the Large Underground Xenon (LUX) experiment in South Dakota—present the results from their searches for dark matter candidate particles called weakly interacting massive particles (WIMPs). Both collaborations attempt to directly detect WIMPs as they scatter off the nuclei of a detector. CDMS finds a signal that could potentially be assigned to WIMPs. But LUX, with nominally better sensitivity, sees no evidence of such dark matter particles.