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Dark matter particles might be even smaller than we thought they were, and even better at interacting with normal matter. If that’s the case, we’ve been hurting our chances at spotting them by burying our detectors kilometres underground.

Nobody is quite sure what dark matter is. The most popular theory says it is probably made of weakly-interacting massive particles (WIMPs), which are relatively heavy but which pass right through normal matter instead of bouncing off of it.

The detectors we use to find those particles are usually placed deep in mine shafts because the rock above them will deflect unwanted background radiation like cosmic rays while allowing dark matter particles to pass through unhindered. But if dark matter particles are able to crash into or bounce off of regular matter, and at the University of Southern Denmark in Odense say that burying our detectors may be a mistake.

Emken and Kouvaris created a supercomputer simulation to examine how light dark matter particles – particles with masses about the same as a proton or lower – could be deflected and scattered as they pass through the Earth.

Making a splash

If these particles do not interact with normal matter, we may never detect them because of their small masses. “They’re too light to create a splash in the detector,” says Kouvaris. “But if it is light and strongly interacting with ordinary matter, it might show up.”

To find these particles, Kouvaris says that we could use detectors above ground to search for daily variations. If dark matter is weakly interacting, there should be about the same amount of it passing through every area within the Earth – but if it’s not, there should be more of it closer to the planet’s surface, particularly on one side because Earth is traveling through space that’s filled with dark matter.

“If you put your hand outside of the car window, there are more molecules hitting your hand from the front than from the back, simply because you are moving against the molecules,” Kouvaris says. “It’s the same thing with dark matter as the Earth moves.”

Every day, as the planet rotates, the “front” of the planet changes. This should cause a daily variation in the amount of light dark matter that we see at any particular detector.

Kouvaris says this daily variation should be highest in the southern hemisphere because of the way the Earth is tilted as it travels through space. This tilt means that when, for example, Australia is at the “back” of the planet, dark matter particles would have to travel through the entire planet’s diameter to reach a detector there. When a European dark matter detector is at the back, the planet’s tilt means that dark matter doesn’t have to traverse so much of the world, so it wouldn’t be deflected as much before it reaches the detector.

Several particles?

There are no current dark matter detectors below the equator, but he says a few detectors there could help either find light dark matter or rule it out as a major component of the the universe. at the University of California, San Diego says it may not be that simple.

“In the next 5-10 years, I think we can easily test a lot of the elements of this kind of strongly interacting dark matter,” says Lin. Even if we don’t find it, though, tiny amounts of these particles could still be out there. “It could always be .00001 per cent of dark matter and we may never be able to rule that out,” Lin says.

“If the 5 per cent of matter that is regular matter is so complicated with so many types of forces and particles, why should dark matter be so simple to be only one particle?” Kouvaris says.

Read more: The sun’s energy could speed up dark matter so we can detect it