There was an interesting question posed by Marcus Ranum about the nature of the WIMPs (Weakly Interacting Massive Particles) that are being looked for as the constituents of dark matter, and which are proving to be so elusive. He wondered why their presence could not be detected via gravity since it was to explain the gravitational effects of galaxies that they were postulated in the first place. I thought the question merited a quick primer for those interested in understanding it in a little more depth.
Part of the confusion surrounding this issue is that the term ‘weak’ refers to both the strength of a force as well as being the name given to one of the four basic forces. All the elementary particles that we know of interact with each other via one or more of the four basic forces: strong nuclear, electromagnetic, weak nuclear, and gravity, listed in descending order to strength. Each of those forces is believed to be generated by the elementary particles exchanging other elementary particles known as ‘gauge bosons’. The strength of the force depends on the strength with which each gauge boson interacts with the elementary particle.
For the strong nuclear force, these gauge bosons are called gluons and are massless. So, for example, quarks exchange gluons and the force thus generated is very strong and increases rapidly with distance between the quarks which is why individual quarks have not been isolated. Quarks seem to be permanently trapped inside larger particles. The proton, for example, contains three quarks, the pion has two quarks, and so on.
For the electromagnetic force, the gauge bosons consist of photons that are also massless. Electrically charged particles exchange photons and this generates the electromagnetic force and while this is strong, the force decreases with distance between charged particles and that is why we are able to overcome the attraction of opposite charges within an atom and get isolated single charges such as electrons and protons.
For the weak nuclear force, we have three gauge bosons: W+, W–, and Z0, that have quite large masses on the elementary particle scale, close to 100 times the mass of a proton. The weak interaction is involved whenever we have neutrinos.
For gravity, the gauge boson is the graviton, which is also believed to be massless though we have not as yet directly detected it because the interaction is so extremely weak that it is only when one of the interacting masses is massively large (like planets, starts, black holes, galaxies, etc.) that the cumulative strength becomes detectable.
So to summarize, gluons couple strongly to particles, photons less so, weak interactions even less so, and gravitons couple so weakly that they have not been directly detected as yet.
The problem with dark matter, as ahcuah pointed out, is that we don’t know what it is made of and so we can only speculate as to what its interactions with familiar matter may be. (Recall that when we say we ‘detect’ a particle in an experiment, what we are saying is that we observed the particle interacting with some particle in the detector. In the LUX experiment, they were looking for the recoil of Xenon atoms as a result of interacting with a WIMP.) We know that because dark matter is believed to be all around us in huge amounts and yet we do not feel it, it cannot be a strong or electromagnetic interaction. We know it has mass so the gravitational interaction plays a role but that does not help us in detecting it because of the extreme weakness of that force. Because it had to have been produced in the Big Bang, we believe it must interact via a force other than gravity so we hope that it interacts via some kind of weak interaction, either of the kind we already know or of a new kind. That is why people are looking for WIMPs.
As is the case in science, we start with what we know and then move beyond. In the early days, it was suggested that WIMPs may consist of weakly interacting particles that we are already familiar with, such as neutrinos, so that the particle and the interaction would be known ones. But that seems to have been ruled out and WIMPs, if they exist, are now believed to a form of matter that is not part of the current known elementary particle spectrum. Various theories that predict different properties have been proposed and what the recent negative results suggest is that some of the ones seen as most promising are becoming less plausible contenders.