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The mysterious substance called dark matter is intrinsically invisible. It cannot be directly observed—rather, its presence is inferred by its gravitational influence on the universe, such as binding galaxy clusters together and moving stars around their galaxy faster than they should.

Yet new research in Â鶹ÒùÔºical Review Letters uses a "camera" to look for dark matter interactions, thereby probing the nature of this elusive stuff.

One hypothesis is that dark matter is made of as-yet unknown particles that are subject to gravitational force but interact extremely weakly with ordinary matter, explains University of Chicago Prof. Paolo Privitera, the spokesperson of the (DArk Matter In CCDs at Modane) international collaboration, which conducted the study.

Over the past several decades, the search for dark matter particles has focused on WIMPs, or Weakly Interacting Massive Particles, believed to be far heavier than a proton.

"But WIMPs have not been found so far, despite extremely sensitive searches by enormous detectors weighing a ton, including my colleague Luca Grandi's work with ," said Privitera.

Experiments at the most advanced particle accelerators, including the experiment at the Large Hadron Collider at CERN, have also failed to find WIMPs.

Astrophysicists are now expanding the search to lighter particles, which requires exceptionally sensitive instruments because signals produced by such low-mass, low-energy particles would be almost impossible to detect.

The DAMIC-M experiment searches for these elusive signals 5,000 feet below the surface of the French Alps. Though it did not find dark matter in its initial run, the experiment was able to rule out several such particle candidates known as "hidden sector" dark matter.

Hidden-sector detector

Dark matter detectors are designed around the premise that dark matter particles will, on very rare occasions, collide with a nucleus in one of the detector's atoms. The recoil of the nucleus may emit light, strip electrons or shake the atom's lattice, producing a signal.

A lightweight dark matter particle is much more difficult to detect than a heavy one.

"A heavy particle hitting a nucleus is like a bowling ball hitting another bowling ball—it will impart a sizable momentum," said Privitera. "A light particle hitting a nucleus would be like a ping-pong ball hitting a bowling ball. It would not move it at all."

However, hidden-sector dark matter would interact with electrons, which are thousands of times less massive than a nucleus.

"Now it is like a ping-pong ball hitting another ping-pong ball," said Privitera.

An instrument sensitive enough to detect single electrons would be ideal to search for hidden-sector dark matter.

The DAMIC-M experiment uses charge-coupled devices, or CCDs, to achieve such unprecedented sensitivity and resolution. Standard scientific CCDs are light-sensitive devices that convert photons into electrical charges, which are then processed into a digital image.

They serve as the "camera" of astronomical telescopes. CCDs are also capable of "imaging" particle interactions, which leave a trail of electrical charges in the device.

DAMIC-M CCDs are much thicker to maximize the detector mass for dark matter particle interactions. The experiment's special CCDs are also capable of , an innovation that allows researchers to count electrons individually. The team looks for pixels or clusters of adjacent pixels with just a few electrons—potentially indicating a dark matter interaction.

These collisions are extremely rare and could be obscured by signals from background sources, such as natural thermal fluctuations in the detector's material. To help minimize this, DAMIC-M CCDs are operated at -220°F.

To mitigate the effects of external radiation, the detector is protected by several layers of shielding. Located at the Laboratoire Souterrain de Modane beneath the French Alps, the detector is sheltered from cosmic rays by more than 5,000 feet of rock. To reduce background from naturally occurring radioactive elements found in the cavern's walls, the CCDs are surrounded by lead.

"Fun fact," said Privitera, "We use ancient lead, from a sunken Roman ship and Spanish galleon, since its radioactive contaminants have already decayed."

For this study, the team built a prototype—the Low Background Chamber, hosting two CCD modules and weighing just 26 grams—and took several thousand "photographs" over two and a half months. They then searched these images for clusters of pixels suggesting dark matter interactions.

The team found 144 clusters with two electrons and only one cluster of four electrons—results compatible with the expected backgrounds.

"Thus, we have not yet discovered dark matter," said Privitera, though he added the results are "orders of magnitude more sensitive than any other experiment, a notable achievement when considering they were obtained with a prototype detector and a small mass."

As the search continues, the absence of a dark matter interaction signal has profound implications for the nature of dark matter.

'Freeze-in' or 'freeze-out'

In one potential, simplified scenario of the universe's evolution after the Big Bang, dark matter and ordinary matter start at equilibrium and transform into each other at equal rates.

As the universe expands and cools, it becomes increasingly difficult for ordinary particles to encounter each other and create dark matter, which requires a high-energy collision.

However, it takes no energy for dark matter particles to meet and destroy each other, turning back into ordinary matter, so the abundance of dark matter would rapidly decrease after the Big Bang. Eventually dark matter particles also become too spread out to engage and the amount stabilizes to what we measure today. This scenario is known as "freeze-out" of dark matter.

In another possible scenario, dark matter particles interact so weakly that dark and ordinary matter are never in equilibrium. On the rare occasion that dark matter is produced by ordinary matter interactions, it does not transform back—and increases in abundance.

The production of dark matter, as with the freeze-out scenario, is limited by the expansion of the universe, so the amount of dark matter eventually stabilizes to the amount measured today. This scenario is known as "freeze-in" of dark matter.

The freeze-in and freeze-out scenarios restrict the properties of dark matter—specifically its mass and interaction probability—and theorists have predicted the properties that hidden-sector dark matter must have to be compatible with the freezing scenarios.

"These theoretical predictions are now probed for the first time by the DAMIC-M null result," said Privitera.

For the freeze-out scenario, a stringent relationship exists between how much dark matter is observed today and its probability of interaction. This constraint allows researchers to make clear predictions of a candidate particle's likelihood of interacting with the detector's electrons and producing a signal.

Because the team did not detect signals, the experiment completely excludes several hidden-sector candidates—they do not exist.

But for the freeze-in scenario, an absence of signal doesn't definitively rule out the existence of that candidate.

"The fact that we have not found dark matter in our data excludes that hidden-sector particles constitute the entirety of dark matter in the universe," said Privitera.

Yet if hidden-sector dark matter exists, it could be a fraction of all dark matter, with something else comprising the rest.

Scaling up

Following the success of the Low Background Chamber prototype, the full DAMIC-M apparatus is set to begin data collection in 2026.

The full-scale detector will have a greater chance of capturing a rare interaction, the scientists said, and the backgrounds will decrease significantly due to better shielding and fewer radioactive contaminants in the apparatus materials.

"Our target is still hidden-sector dark matter, which we may find composing a fraction of all dark matter, but also light WIMPs and other candidates," said Privitera. "We expect that DAMIC-M will be the leading experiment in the search for these low-mass dark matter particles for several years to come."