There really is something out there, bending the light from distant galaxies:
Galaxies, and galaxy clusters, appear to be surrounded by clouds of something invisible which interacts gravitationally. It apparently also causes the rotation of galaxies to deviate from the simple prediction of Newton's laws assuming that only the visible matter in galaxies is present. This "dark matter" makes up nearly a quarter of the mass/energy density of the universe, whereas "light matter" (stars, interstellar dust and atoms) make up only a few percent. Our prejudice is that dark matter has a particle nature. Theories of particle physics beyond the Standard Model these days offer many possibilities for dark matter candidate particles, many of which could be detected via their weak interactions with ordinary matter. Here we mean weak in the sense of interacting by exchanging a W or Z boson with the ordinary matter particles. The common thread among the particles proposed for dark matter is that they are weakly interacting and massive (of the order of hundreds of times the proton mass - one to several hundred GeV). Particles in this general class of dark matter candidates are called WIMPs. If WIMPs are really there, there are only a few per cubic meter in our galaxy, and most fly straight through ordinary matter without interacting. But, if they can and do interact weakly, occaisionally they could transfer enough energy to a nucleus of ordinary matter that it would be detected, if our equipment is sensitive enough. Also, there is hope that WIMP dark matter particles could be created in the high energy collisions of the Tevatron at Fermilab or soon at the LHC at CERN, and detected indirectly by the fact that they carry away apparently missing energy. Experimentally, now, in the area of direct detection there is a race on between two main competing technologies for observing the feeble signal of WIMP interactions. Until a few days ago, the most sensitive search for WIMPs was that of the XENON10 collaboration, using a detector of liquid and gaseous xenon as the target for the WIMPs. The detector is kept deep underground at the Gran Sasso Laboratory in Italy. But at the Dark Matter 08 meeting in Marina del Rey last week, the lead in this search was recaptured by the Cryogenic Dark Matter Search collaboration. They use solid crystalline silicon and germanium detectors, cooled to liquid helium temperatures, to sense the nuclear recoil from WIMP interactions, taking extraordinary measures (as all these experiments do) to avoid false signals from cosmic rays, natural radioactivity, and stray neutrons. They did a blind search, making carefully controlled predictions of the number of WIMP signal events they expected to see, and then "opened the box" earlier this month. They expected 0.6 background events, give or take about half an event. The result: nothing. No signal from WIMPs or backgorund or anything! To "measure nothing" is usually a great experimental challenge. You do have to convince the world that you would have seen something if it had been there, that your apparatus isn't just mute for some other reason. CDMS have done a great job convincing the world of this, I'd say, and their result is nearly a factor of three more constraining than the previous Xenon 10 result. They show it in this plot of WIMP interaction strength versus WIMP mass:
What the plot is saying is that assuming that all dark matter is a WIMP of a certain mass, there is less than a 10% chance that, if the spin-independent cross section had some value greater than that indicated by the heavy black line, they would have seen no events in the detector. This can be turned around to say that at the 90% confidence level if such WIMPs exist, they must have even feebler interaction strengths than that indicated by the heavy black line. These exclusion limits are starting to cut deep into the theoretically favored regions indicated on the plot...and the fact that there really are zero events at this stage means that any sort of five sigma discovery could be a long way away. In the mean time, there are larger xenon based experiments planned, including a larger version of the Gran Sasso one and the LUX experiment, to be based at the nascent deep underground facility DUSEL. And, perhaps as early as later this year, the LHC experiments CMS and ATLAS will begin to get a glimpse of the results of protons "colliding in anger" as a colleague of mine likes to put it. The Tevatron has seen no hint of WIMP production/decay yet, and so with seven times more energy, the LHC maybe able to produce the heavy particles that decay down to lighter ones which may include WIMPs such as the neutralinos predicted by supersymmetry. I muse from time to time on the possibility that dark matter may really only interact with "light matter" gravitationally. Perhaps it is composed of an entirely separate sector which does not interact with light matter. It may be several or many particles which interact amongst themselves, forming structures we can only speculate about. If our only probe of dark matter is gravity, there will be a very long road ahead to understand it. I hope the cosmologists among the readers here can offer us more hope that indeed, there are strong reasons to believe dark matter interacts weakly...
