"THEIR INVENTORS" GIVES THE WHOLE GAME AWAY:

IT'S a troubling time to be looking for the universe's missing matter. On the face of it, it shouldn't be. Deep underground, several experiments have been buzzing with possible sightings of dark matter, the hitherto invisible stuff that is believed to make up around 85 per cent of all matter in the cosmos. Detecting dark matter would be a major triumph.

Yet any hopes that the nature of the stuff would be quickly revealed by these first detections have been utterly dashed. The trouble is that dark matter appears to be different things to different detectors. It appears heavier in one detector than another; it appears more ready to interact in one experiment than another. In the most extreme case, it shows up in one instrument but not in another - even when both are made of identical material and are sitting virtually next door in the same underground lab.

"The present situation is pretty confusing," admits Juan Collar of the University of Chicago, who is head of the CoGeNT dark matter experiment, based in the Soudan Underground Laboratory in Minnesota. [...]

Then there's the new kid on the block that some are claiming could do the job just as well: "dark atoms". Not only could dark atoms explain the lack of dwarf galaxies, it's just possible, say their inventors, that they could also explain the discrepancies between the dark matter experiments. [...]

What's promising about dark atoms is that they could explain the lack of dwarf galaxies in our observations. Because dark atoms would emit or absorb dark photons, the universe might be full of invisible, dark light that constantly interacts with clouds of dark atoms, raising their temperature and puffing them up. This would prevent dwarf galaxies from forming in the first place. "It's still a rough, back-of-the-envelope calculation at the moment," admits Wells, who has started working on simulations to better test the idea.

Still, many researchers are not quite prepared to abandon WIMPs yet. "I don't find the arguments about dwarf galaxies very convincing," says Dan Hooper of the Fermi National Laboratory in Batavia, Illinois. "My money is still on WIMPs."

He suggests that the missing dwarf galaxies could be out there but are invisible because they are made solely of dark matter. One way to find them is to look for gamma rays, which should be produced when WIMPs collide and disintegrate. NASA's Fermi space telescope has searched for such gamma rays in nearby dwarf galaxies twice and so far found nothing. That does not necessarily mean that the WIMPs are missing, just that they are not in the form expected. And Hooper says that the results do not rule out low-mass WIMPs similar to those that may have been seen in the underground experiments.

Collar dubs the current impasse "a world of pain" and reckons that it is likely to get worse before it gets better. Now that we have begun to see something, either astrophysics is wrong, or particle physics is wrong, or our whole understanding of dark matter is wrong.

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