Dark matter is kind of old-hat in these exciting days of dark energy. Nevertheless, we don't know very much about it. We know that it exists, we have a very good idea of where it is, and we even know how much of it is around—but we don't know what it is or how it interacts with other matter aside from via gravity. These are now central questions for those who study dark matter, but they are also expected to be a short-lived ones. We can expect some pretty good answers in the next ten years as the LHC begins to build up a large database of collisions and data comes in from many cosmological observations that are already in progress.

The cosmological observations are already raising eyebrows and causing theorists to sharpen their pencils, though. All of these observations find significantly more electrons and positrons than expected coming from the galactic center. Now, in a very cool piece of physics, researchers have shown that all of these observations can be explained by dark matter interacting with itself.

First, lets take a look at the observational data, starting with the cosmic microwave background radiation. The current map, produced by WMAP, shows a haze of hard radiation around the galactic center. It turns out that the data is best explained by synchrotron radiation, produced by charged particles going around in circles near the galactic center. But that implies that there are more charged particles out there than we'd expect.

Then there is the data obtained by PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics). The scientists in charge have found that there are many more high-energy electrons and positrons than can be accounted for by the interaction of high-energy cosmic rays with the interstellar medium—they have already accounted for all other known sources in order to study cosmic rays in the first place. Similar results have been found by other cosmic ray observatories, and by gamma ray observatories as well.

There are other observations that seem to display the same sort of anomaly but—and here is the kicker—all of these experiments are observing different physical phenomena in different ways, yet reporting the same problematic result. To add to cosmologists' problems, the conflicting results have been obtained by the LIBRA/DAMA collaboration and their competitors. LIBRA/DAMA claims to have observed an annual modulation in dark matter collisional interactions, a result they ascribe to the Earth traveling with and against galactic rotation as it circles the sun. Similar experiments run by other groups have failed to find such a signal, and the contradictory results have, frankly, confused everyone.

To make matters worse, every single observation could be explained by assuming that something is unusual about that particular observation—for instance, supernovae could be blurring WMAP's vision. But this is pretty unsatisfactory, because the point of observation is to collect phenomena under an umbrella of a few descriptive and predictive models, rather than adding extra phenomena to explain each observation.

A group of scientists have taken the first steps toward attempting to unify these phenomena under a single theoretical umbrella. To do this, they have posited that the electrons and positrons are the result of dark matter annihilations, while other annihilation paths that lead to different particles are suppressed. This idea is not as obvious as it sounds, because the number and energies of the electrons and positrons indicate that dark matter must be pretty strongly interacting, and interacting is on the list of things dark matter doesn't do.

This problem can be alleviated if one assumes that there is relatively long-range force that acts between dark matter particles. This force can then enhance the probability that dark matter annihilates in a way that produces electrons and positrons while also suppressing other annihilation pathways. The nice thing is that this force provides an internal structure to dark matter that also explains why DAMA/LIBRA saw a dark matter annual modulation signal and other experiments did not. In other words, this single hypothesis brings a wide range of observations together and makes predictions about the properties of dark matter—everything you want in a hypothesis looking to make its way to theory status.

But, I hear you say, they have replaced a few special phenomena that we know exist with one force that we don't know exists—surely, that is a step backwards. Well, it is true that further examination may prove that some of the observations are the result of special circumstances. It is, however, unlikely that all of them are. Furthermore, we know that there are very likely to be other bosons out there, awaiting the LHC attempt to reveal their existence. It doesn't seem unreasonable to suppose that one of them has the mass required for a dark force carrier.

So, no, this isn't pure speculation, and, no, it doesn't rely on "something really strange emerging from the LHC." Nevertheless, it still requires a lot of confirmation before it gets its own chapter in physics text books.

Physical Review D, DOI: 10.1103/PhysRevD.79.015014