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For more than 20 years, an experiment buried beneath a mountain in Italy has spied a strange signal that the researchers running it believe to be a sign of elusive dark matter particles. Now, a smaller but otherwise nearly identical experiment in Spain has failed to find this signal, leading scientists to question whether the Italian detector really saw anything at all.

Both detectors are designed to search for weakly interacting massive particles (WIMPs), a front-runner to explain the effects of dark matter. These particles are hypothesised to be similar to the standard particles of physics, but very rarely interact with normal matter, passing right through it most of the time. If WIMPs exist, they should suffuse our galaxy.

The two detectors – the DAMA/LIBRA experiment in Italy and the ANAIS experiment in Spain – both use sodium iodide crystals to search for WIMPs. As the particles pass through the crystals, they should bump into a few nuclei, causing them to emit light that the researchers can detect.

Because of Earth’s orbit around the sun and the solar system’s orbit around the centre of the galaxy, the amount of dark matter entering the detectors should go through an annual cycle. This cycle is what the DAMA/LIBRA researchers claim to have found, but ANAIS hasn’t seen any sign of it so far.

“Right now, ANAIS does not see it, so this is a sign that it is not dark matter,” says María Martínez at the University of Zaragoza in Spain, who is part of the ANAIS team. “But our amount of data right now is much, much less than DAMA/LIBRA – they have been measuring for 20 years and we have 3 years so far.” It will take more data to confirm the ANAIS results, but the recent non-detection is a discouraging hint.

That isn’t necessarily surprising, given that other experiments have been inconsistent with the DAMA/LIBRA findings. “I think these null results are what most of us expected — theorists tried all manner of theories to fit the data, and all of them were in tension with other existing… experiments,” says Kathryn Zurek at the California Institute of Technology.

However, the results from these other experiments couldn’t be directly compared to the DAMA/LIBRA results because those detectors didn’t use the same materials and methods, so all of the comparisons required assumptions about the behaviour of the detectors. “These experiments, for the first time, are making apples-to-apples comparisons rather than apples to oranges,” says at the University of Texas at Austin.

Rita Bernabei at the Tor Vergata University of Rome in Italy, a DAMA/LIBRA spokesperson, contends that the two experiments and the teams’ methods of analysing their data are different enough that ANAIS may be unable to detect the same dark matter signals as its Italian forebear.

“No one has doubt that there is an annual modulation in the DAMA/LIBRA data; the issue is whether the annual modulation is dark matter or something else,” says at the University of Zaragoza, also part of the ANAIS team. The ANAIS results hint that it isn’t dark matter, but rather some sort of systematic error in DAMA/LIBRA, she says. Several similar experiments around the world are already underway, so the coming years may bring more solid proof for WIMPs – or send dark matter hunters back to the drawing board.

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