Physicists currently believe that most of the dark matter in the universe is made up of individual particles, and the challenge is to work out what kind of particles these are. New research, however, overturns this assumption and says that observational and experimental data are better explained if dark matter exists as composite particles – atoms of dark protons and dark electrons that are acted on by the dark-matter equivalent of the electromagnetic force.

Dark matter is thought to make up more than 80% of the matter in the universe. As its name suggests, dark matter does not reveal itself by emitting light because it does not interact via electromagnetism. Its existence is instead inferred through its gravitational effects on normal matter.

Physicists' favourite candidate for dark matter is a broad class of so-called weakly interacting massive particles, or WIMPs, which interact via the weak nuclear force. WIMPs are in line with much of the observational evidence for dark matter, but two anomalies remain. One is the fact that WIMP models predict that dark matter ought to clump together gravitationally at all length scales, from galaxies down to much smaller sub-galactic structures. However, this is not what is observed – no dark-matter structures smaller than about 400 light-years across have been found by astronomers.

And then there's DAMA

The other problem concerns the results of experiments on Earth designed to detect dark-matter particles directly via their collision with nuclei of ordinary matter. One such experimental collaboration, DAMA in the Gran Sasso laboratory in Italy, has generated controversy by claiming to have collected extremely strong evidence for dark matter inside its detector. Unfortunately, DAMA's results cannot be interpreted as collisions of WIMPs without appearing to strongly contradict a number of other experiments around the world.

Now, David Kaplan and colleagues at Johns Hopkins University in the US say that these two problems could be overcome if dark matter consists not of individual fundamental particles but is instead largely made up of composite "atoms". These atoms would be made up of the dark matter equivalent of protons and electrons bound together by the equivalent of the electromagnetic force, and would be accompanied by a certain fraction of ionized atoms – in other words, free electrons and protons.

The researchers point out that the existence of these charged particles would have altered the evolution of dark matter in the early universe. WIMPs, being uncharged, would have decoupled from normal radiation less than 1 second after the Big Bang, whereas atomic dark matter, with its ionized fraction, would have remained in thermal equilibrium with dark radiation for about the first 20 minutes. The universe would therefore have expanded to a certain size before gravitational clumping could have occurred, dictating the size of the smallest dark-matter structure that we see today.

Inelastic collisions

To explain the discrepancy between DAMA and other experiments, Kaplan and colleagues build on an idea put forward by Neal Weiner and David Tucker-Smith in 2001. Weiner and Tucker-Smith proposed that the collisions detected by DAMA are inelastic, that some kinetic energy is lost because on collision the dark-matter particles absorb energy to become slightly more massive and that these energy-sapping collisions are far more likely to occur with the relatively heavy sodium iodide in DAMA's detector than with, say, the silicon and germanium of which CDMS detector in the US is made of. Kaplan's group, on the other hand, says that this energy loss can be explained by the incoming dark-matter atoms jumping up an energy level when they collide, rather than being due to the creation of new particles that are postulated specifically for this process.

The researchers admit that there is a "tension" within their model because the explanation of missing structure in the universe requires a higher fraction of dark atoms to be ionized than does the mismatch of experimental results. But they say that this difference can be resolved if atomic and ionized dark matter assume different halo shapes within galaxies.

Kaplan's colleague Christopher Wells admits that their proposal is speculative but that it does have the additional benefit of bringing dark matter more into line with the ordinary matter that we are familiar with. Indeed, they say that dark hydrogen atoms could bind to form hydrogen molecules and that the formation of these molecules could then lead to the creation of "dark stars" or other compact objects. They add that the interaction of dark photons with ordinary photons could lead to emission lines in the spectra of cosmic gamma rays.

Not really problems?

Daniel Hooper, an astrophysicist at Fermilab in the US, does not believe that the problems being addressed by the atomic dark-matter model are really problems at all – that the problem of structure formation is essentially solved while the DAMA results are "not very compelling". "That being said," he adds, "to those scientists who think these are issues that need solutions, the 'atomic dark matter' idea presented here does seem to resolve the problems fairly easily."