“WHEN you have eliminated the impossible, whatever remains, no matter how improbable, must be the truth.” Though that maxim of Sherlock Holmes would rarely withstand scrutiny in the everyday world, where facts can be fuzzy and the truth is often protean, it is not a bad one for fundamental physics—a field where there really is only one right answer. It has certainly been the approach taken by Dan Hooper and Lisa Goodenough, two hunters of some of physics’s most elusive creatures: the particles of which dark matter is composed. They think they have eliminated all alternative explanations to these particles being the origin of a powerful clutch of gamma rays that come from the centre of the Milky Way, the Earth’s home galaxy, and they have been saying so for several years.

This week the chief remaining group of sceptics—the team that runs the satellite which detected the gamma rays in question—has thrown in the towel and agreed that it, too, can come up with no convincing alternative. Though this concession does not, quite, close the “Case of the missing WIMPS”, it will require a considerable reversal of fortune for Dr Hooper and Dr Goodenough now to be proved wrong.

The WIMPS in question are weakly interacting massive particles, the name given to the putative components of the mystery that is dark matter. Exactly what WIMPs are is not clear. What is clear is that they have gravitational attraction, but otherwise interact only vanishingly rarely with the more familiar matter of which atoms are made. Nor do they interact with light. Dark matter is thus invisible. Everything from the motions of galaxies to calculations about what sort of universe came out of the Big Bang says it must exist—and must outweigh familiar, atomic, matter by about six to one. But no one has ever detected it other than by its gravitational effects. Dr Hooper and Dr Goodenough think they have found a second way. They believe that the gamma rays they have been analysing are flashes of radiation given off when WIMPs run into one another.

Spot marks the “X”

The satellite which collects these flashes is Fermi, an American space telescope launched in 2008. Gamma rays (electromagnetic radiation like light, but much more energetic) are generated by the universe’s most violent processes—the leftovers of supernovae, particles accelerated in potent magnetic fields, matter swirling ever faster around black holes and so on. Data on the rays Fermi detects are published unedited, so that any scientist who wishes can poke through them.

Among those who poked were Dr Hooper (who works at the coincidentally named Fermi National Accelerator Laboratory, near Chicago) and Dr Goodenough (then a graduate student at New York University; now at Argonne National Laboratory, also near Chicago). They looked in particular at rays from the centre of the Milky Way. As is true of any galaxy, much of the Milky Way’s matter—including its dark matter—is concentrated at its centre. This therefore seemed, to Dr Hooper and Dr Goodenough, to be a good place to look for signs of colliding WIMPs.

Unfortunately, the amount of matter at the Milky Way’s centre means there are many other sources of gamma rays, too. The two researchers had therefore to subtract from the data collected by Fermi all of the radiation they thought could be accounted for by cosmic objects already known to astronomers. What they were left with (see picture above, of a computer-generated image of the gamma rays superimposed on a visible-light photograph of the Milky Way), was a bright residuum of gamma radiation that would be inexplicable if it were not caused by dark matter.

Dr Hooper and Dr Goodenough first observed this surplus six years ago, and made it public then, so that other physicists could attempt to refute it. Nor was their claim the only one of its kind: the scientific literature abounds with reports of intriguing blips spotted in Fermi’s data by enterprising researchers. But, while other claims to have seen dark matter in these data have fallen away, explained by ever-better models of the underlying physics, or dismissed as artefacts created in the telescope’s machinery or in the maths needed to subtract the known from the unknown sources of radiation, theirs has not.

The phenomenon they see falls off in intensity away from the Milky Way’s centre exactly as would be expected were dark matter responsible. And the frequencies of the gamma rays match what several plausible-looking models predict will happen when WIMPs cross paths: transmogrification into familiar-matter particles called bottom quarks, with the balance of the energy involved speeding off as gamma rays of exactly the sort Dr Hooper and Dr Goodenough have seen. The truth of their observation is therefore now widely enough accepted in dark-matter circles that the putative particle responsible has, playfully, been dubbed the “hooperon”.

The endorsement Dr Hooper and Dr Goodenough really wanted, though, was from the Fermi team itself, which knows better than anyone else what the telescope’s kinks are, and thus how artefacts might be created. On November 10th that endorsement came—with the release of an analysis to be published in the Astrophysical Journal, led by Simona Murgia of the University of California, Irvine, who helps run the telescope’s main detector.

Dr Murgia says Fermi’s operators—who were, of course, able to see the data as they arrived, and thus steal a march on outside analysts—knew about the anomalous surplus even before Dr Hooper and Dr Goodenough, and have been working ever since to try to explain it away. They kept shtum because, she explains, grand international collaborations like Fermi are conservative by their nature. “When you see something you expect, you write a paper,” she says. “When you see something you don’t expect, there’s a lot of discussion and arguing and it’s not always helpful. In this case it delayed things a bit much.”

On being a WIMP

There are still a few die-hards who do not believe in hooperons. They suggest that if an ensemble of millisecond pulsars (dead stars that rotate hundreds of times a second) were buried in the Milky Way’s middle, that might do the trick. To see off this theory (or indeed, prove it correct) requires further lines of evidence.

One such may be dwarf galaxies—conglomerations of just a few billion stars (as opposed to the hundreds of billions in galaxies the size of the Milky Way) that are believed to host lots of dark matter and few confounding gamma-ray sources. Fermi can peer at those too, and has. Indeed, gamma rays it has detected coming from one such galaxy, Reticulum II, may support the Hooper-Goodenough hypothesis. Another possibility is that an intriguing signal noticed by XMM-Newton (a space telescope that scans the skies for X-rays rather than gamma rays), which has defied preliminary attempts at conventional explanation, might turn out to be a consequence of dark matter.

Dark matter might also be caught more directly, in vast underground laboratories on Earth. These employ lumps or tanks of special materials dedicated to detecting the flash of light or heat that occurs on those rare occasions when a WIMP actually does deign to interact with an atomic nucleus. The sensitivity of such experiments has improved markedly in recent years, so WIMP fans remain hopeful.

And WIMPs may even show up in the Large Hadron Collider, a particle accelerator in Switzerland which, after a long upgrade, should now be powerful enough to spot evidence of dark-matter particles in the detritus of its smashings.

What is at stake for all these efforts is tremendous. If and when any of them does confirm the suspicions of Dr Hooper and Dr Goodenough, a new and odd inhabitant will have to be admitted to the zoo of particles known as the Standard Model; a new taxonomy will have to be developed to accommodate it; and particle physicists will have six times more stuff to study than they had before.