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One of the world’s leading dark matter detectors has wrapped up a nearly two-year-long search for the mysterious particles, without finding a single whiff. The results suggest that the days may be numbered for the dominant model of dark matter.

We’ve known since the 1930s that without dark matter‘s gravitational pull, galaxies would spin themselves apart. This mysterious substance, which does not emit light or interact with normal matter except through gravity, should make up around 85 per cent of the universe’s mass.

After ruling out ordinary matter that just doesn’t emit much light, theorists settled on some basic characteristics for their quarry: it should be made up of particles that have some mass and interact weakly with other matter. They called them “weakly interacting massive particles”, or WIMPs, and set about building detectors that could catch them.

What’s unknown is how often these particles bounce off each other – their scattering cross section – and their mass. They should also occasionally bump into normal matter. These rare collisions are what experiments like the (LUX) are designed to pick up, in order to determine WIMPs’ properties.

But today at the  in Sheffield, UK, the LUX team announced their final 20-month run, from October 2014 to May this year, ended without a single dark matter detection. That means LUX has ruled out a large number of possible cross sections and masses for WIMPs – to the point where some physicists argue it might be time to abandon the idea all together.

“I think we are getting to the point where the limits are excluding so much of the parameter space that we should rethink,” says  at Harvard University. “Perhaps the dark matter is not WIMPs.”

A decade left

 at Brown University in Rhode Island, who works on LUX, says there is still much more room to explore. Plans are already underway for an that will be 70 times more sensitive.

“That’s a lot of new models you can test,” he says. “We keep knocking over models, and right now any one of those could turn out to be the correct one.”

Gaitskell says the experiment’s technology, which detects flashes in pools of liquid xenon when they are hit by a WIMP, is improving faster than Moore’s law – the rate at which the number of transistors on a computer chip increases. That’s a good sign for those hoping we can still catch WIMPs – the next generation of detectors should be exponentially better, not just a bit better.

“To avoid everybody dying of boredom and running out of money, you have to do it as fast as you can,” he says. But that means in a decade or so, WIMPs will be out of hiding places. “On a 15-year view you have to be ready to admit that, if we fail to see anything.”

Another ding against WIMPs is the lack of any new particles at the Large Hadron Collider, which could offer hints at how dark matter interacts. It has spotted signs of something with a mass of 750 gigaelectronvolts, but as it is produced by two photons colliding, it probably isn’t related, says Loeb.

“That’s the only surprise at the LHC,” says Loeb. “I wouldn’t connect it to the dark matter.”

Now what?

So if dark matter isn’t WIMPs, what is it? There are no shortage of alternatives waiting in the wings. Light-weight particles called axions are one option, while tiny black holes left over from the big bang are another.

Some renegades want to do away with dark matter altogether. at the Weizmann Institute in Rehovot, Israel, has fought a 30-year battle to explain the need for extra matter in the universe by instead changing the way gravity works on galactic scales, a theory called modified Newtonian dynamics (MOND).

“I am certainly not surprised when I see negative reports coming from dark matter search enterprises,” says Milgrom, but he would like the search to continue.

“Not finding dark matter at higher and higher sensitivity will only strengthen the case for MOND,” he says. “Not looking for it at all will not help, and may lead to a standoff.”

WIMPs have endured in part because theorists are unwilling to give up on decades of research built up around them – it’s always possible to tweak your model and save it from the latest experimental data, says Loeb.

“The good news about physics is that experiments set the agenda,” he says. “Theorists that have no connections with experiments miss being wrong, and that’s not physics in its actual sense.”

Gaitskell is no stick-in-the-mud – he quit a previous experiment, the Cryogenic Dark Matter Search, when it seemed like it wouldn’t be able to deliver evidence for WIMPs. But he says it’s too soon to count WIMPs out altogether.

“People aren’t always ready to say to themselves, the scientific question we’re choosing to answer could take an entire lifetime,” he says. “I don’t think at this stage there has been any build up in the science that says it’s not WIMPs. I think it’s more that people would rather talk about something new.”