Dark matter is the physical mystery of our time. We know from many different results that a fair percentage of the Universe consists of matter that doesn't seem to interact in any way other than gravity. But with every statement like that, there is a limit: our measurements are only so sensitive, which leaves space for dark matter to interact. If it does, the interactions have to be weak. Hence, most physicists think that dark matter consists of weakly interacting massive particles (WIMPs).

Not all candidate dark matter particles fit with experimental data, so they can be discarded, right? Maybe not, according to physicists at Harvard. "Wait," I hear you say, "data is king, so they can't be right." But they actually can, because when data gets turned into models, assumptions often get involved. The underlying reason for kicking dark matter proposals out is the assumption that the entirety of dark matter consists of a single type of particle (or, more accurately, many different types of particles with very similar properties). But who is to say that dark matter doesn't consist of a mix of different particles with vastly different properties?

Setting off for the shores of a universe that has a complex distribution of dark matter particles begins with a journey of small steps. In this case, the researchers considered a population that has a mixture of standard WIMPs and a population of dark matter that doesn't interact with ordinary matter, but does interact with itself. Think of this second population as a mirror universe of protons and neutrons that can clump together through electromagnetic-like forces but are almost untouchable by ordinary matter. A universe composed of these two populations of dark matter will look very different from one composed of only WIMPs.

There is some room to produce a more familiar universe—as long as there isn't too much of this new dark matter, the universe won't mind—so the researchers calculated just how much dark matter could be strongly interacting. They found that this comes out to about five percent, which is roughly the amount of ordinary matter that is in the Universe. An interesting coincidence, if nothing else.

Interacting dark matter has observational consequences, though, which is what makes it interesting. In a spiral galaxy like the Milky Way, it will accumulate in a disk. That means that our dark matter detection experiments, like Fermi and PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics), should see more dark matter than expected. And although the results are uncertain, both Fermi and PAMELA have reported possible dark matter signals that are stronger than expected.

But the dark matter disk doesn't have to be oriented in the same plane as ordinary matter. So the expected excess experienced by an observer in a galactic disk can vary from zero upward to some maximum amount, depending on the relative orientation of the ordinary matter and dark matter disks, adding yet another free parameter into the mix. This, I think, is a bad thing.

This richer model of dark matter may really come into its own, though, in explaining some odd results. For instance, the bullet cluster observations can be explained by either standard WIMP dark matter or by a mixture of different dark matter components. But the Abel 520 cluster observations are not so easily explained by WIMPs alone, though a more complex version of dark matter might make sense of it.

Unfortunately, we don't yet know. This paper was more about the bounding possibilities: how much freedom is there in the mixture? We would still need a more developed model before dark matter distributions can be compared to real astronomical observations. But this model is timely, because the Planck and Gaia missions will shine an awful lot of light on dark matter.

So where does that leave us? Well, we still don't know what dark matter is, but this paper shows that the possibilities are much wider than the ones we've generally considered. In some ways, that's a lot of fun, because it opens data to new interpretation and should result in a more consistent model. On the downside, our limited observational power may mean that we have to wait a long time to arrive at a unique description of dark matter.

Physical Review Letters, 2013, DOI: 10.1103/PhysRevLett.110.211302