XMASS-I under construction in Japan
The search for dark matter — the stuff that seems to make up most of the mass in the Universe, but which is invisible to us — is loaded with new ideas, tantalising hints, and incredibly advanced experiments. Unfortunately, none of science's best efforts have yielded any definitive proof of dark matter's identity.
One recent dark-matter-hunting experiment in Italy, called DAMA/LIBRA, recently reported a strange, annually occurring signal they thought might be coming from dark matter. But other experiments, including XMASS-I in Japan and the XENON 1T detector in Italy, can't seem to find the same signal — or any dark matter signal.
"An annual modulation signal due to the Earth orbiting around the Sun would be one of the strongest indications of the direct detection of dark matter," the authors from the XMASS-I experiment in Japan write in the new paper published in Physical Review Letters. "We also did not find any particular periodicity in the data," meaning they could not recreate the DAMA/LIBRA results.
Dark matter is the name scientists have given to an effect seen throughout the universe: The way stars orbit in galaxies and the universe's large-scale structure suggests there's around six times more mass in galaxies than scientists actually observe. What that mass is, no one knows yet. Perhaps the most-studied of dark matter's proposed identities would be particles that just barely interact with regular matter, like a breeze on a skyscraper.
Scientists have therefore built quite a few super-sensitive experiments to find these dark matter particles. No one has found them, yet, unless you count the strange signal that won't go away from the DAMA/LIBRA experiment in Italy. Some dark matter experiments, including XENON-1T and LZ, use super-cold liquid noble gases, like xenon, to try to detect tiny hits from dark matter. DAMA/LIBRA uses sodium iodide crystals, which would flash if they were struck by dark matter.
DAMA/LIBRA researchers report that they have spotted an annually modulating signal, which could mean the Earth is passing through more or less dark matter based on its position around the Sun. Imagine a sheet flapping in the breeze — it moves differently depending on whether it's parallel or perpendicular to the wind. DAMA/LIBRA's scientists think they have spotted the dark matter breeze.
But there's a lot of scepticism about DAMA/LIBRA's results. The research team doesn't share their data with other physicists, and no one else has seen the signal. Now, researchers at XMASS-I, a detector with 832kg of liquid xenon in Japan, have reported that they are unable to observe the DAMA/LIBRA signal in a new paper.
Maybe there's something specific about DAMA/LIBRA that's allowing it to catch the signal, or maybe the team is observing something that isn't dark matter. No one else has observed such a signal, but others are now building experiments with similar architectures that will try. That includes COSINE-100 in South Korea, ANAIS in Spain, and SABRE in Australia, as Nature (and Gizmodo) have previously reported.
"We will need to wait for a couple more years to see if the replications actually detect the same level of annual modulation," Yale astrophysicist Pryamvada Natarajan, who was not involved in the experiment, told Gizmodo. DAMA/LIBRA's spokesperson has not yet responded to Gizmodo's request for comment.
This is just one of several "no dark matter" results released this week. The well-known and most sensitive XENON-1T experiment has once again released null results — they found nothing in their hunt for dark matter particles. Expect more about that experiment from us soon.
If a team of scientists really has measured a dark matter signal, others scientists need to be able to recreate the results. Until then, it's safe to say the search for dark matter is turning up a whole lot of nothing.