Australia’s Dark Matter Quest

Dr Katherine (‘Katie’) Mack was sitting in her Melbourne office one afternoon and received an email in November 2013. Katie, a theoretical astrophysicist researching cosmology and dark matter, was at the time working as a postdoctoral researcher at the University of Melbourne.

The sender had recently read an article in The Economist. Katie had provided a few quotes in the article which detailed how certain types of hypothesised dark matter particles could be detected by building specialised, sensitive instruments deep underground and inside old, abandoned mines - where they would be shielded away from a lot of the background radiation that rains down on Earth from space (such as cosmic rays), as well as a lot of the activities on the surface by us busy, yet noisy, humans.

In time, that email would go towards building one of Australia’s most ambitious physics projects because, as fate would have it, the sender happened to be part of a firm that was running a project with an old, disused gold mine in Stawell, a small regional town in the southern Australian state of Victoria. They were pondering what to do with it when his interest piqued, reading The Economist article.

“I really had no idea what to make of it when I received that e-mail. My first thought was, ‘I’m a theorist — I have no idea how to build an experiment,’” said Katie. “I think I just sat on the e-mail for a week, not even knowing if I should take it seriously. But then I realised there were people in the department for whom it might be incredibly valuable.” 

By coincidence, a particle physics research group within the University of Melbourne who study dark matter were also, by chance, in the market for an old, disused mine - searching for an underground location that was isolated enough to build a dark matter detector within it, here in Australia.

“We were all down at the coffee shop one day when the experimenters were talking about their plans, and I said something like, ‘So, I got this weird e-mail…’” said Katie. 

As the saying goes … sometimes, the stars align.

Fast forward several years to July 2025. Deep beneath the same goldfields of regional Victoria, the ARC Centre of Excellence for Dark Matter Particle Physics is in the final preparations, nearing the time when they will turn on the unique instruments at the Stawell Underground Physics Laboratory (SUPL). 

This underground facility, located approximately 1 kilometre below the surface, is shaping up to be one of Australia’s most exciting experimental facilities - unlike any other across the continent, or indeed the southern hemisphere. It represents a melding of Aussie mining heritage, cutting-edge physics and the ambition to probe the Universe’s most enduring mysteries.

“SUPL is a new, world-class facility, established to support breakthrough and applied research requiring an ultra-low background radiation environment,” said Kim Mintern-Lane, Chief Operating Officer of SUPL.

 “The foundational experiment in SUPL, and the initial motivation for constructing the facility, is the SABRE South dark matter experiment, which seeks to detect and characterise dark matter.”

“SUPL is a national research facility for experiments that require, or would benefit from, its unique low background environment, such as quantum technology, dark matter detection, nuclear and background radiation detection, as well as those that benefit from the isolated, controlled, and stable underground environment, both within the laboratory and potentially through the mine’s tunnels.”

SUPL is supported through the partnership of industry and science. Its six member institutions are a collaboration featuring the University of Melbourne, Swinburne University of Technology, the University of Adelaide, Australian National University, the University of Sydney and the Australian Nuclear Science and Technology Organisation (ANSTO). All working together with the Stawell Gold Mine - an active commercial mine site. That is, while the science is occurring or while the dark matter detector is being developed, miners continue with their daily activities of extracting rock through underground blasting (twice per day) and hauling ore back to the surface, where it is refined and treated to extract the gold.

In July this year, Katie and I were lucky enough to visit SUPL organised by the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), with Kim as our guide, who showcased how far along the project has come. From the moment we started the tour, safety was the central focus for everyone - after all, we were descending 1025m below the surface into an environment that is dark, hot, dusty, extremely humid, and requires ventilation.”Other worldly” is a fitting description. As Kim put it to us, there was so much beauty in the many shades of grey in the rock.

Before we set off from the surface, we donned the required safety gear (helmet, glasses, high-vis vest, boots, and a rebreather) and then huddled into a 4WD with Kim at the wheel. This was no ordinary drive - as soon as we started the car, Kim was radioing back and forth codes about our location. The mine's narrow passages and low-level lighting conditions demand constant radio communication with other vehicles, all making their way around like busy ants in the intricate maze of tunnels below our feet. Every corner we took was logged in code, an underground language of safety. 

As our vehicle entered the main access tunnel that leads to the capillarous network of utilised and former mine tunnels, we left the bright, natural light of day behind, replacing it with the artificial beams of the 4WD in the direction of travel as we descended into the darkness. The first impressive experience is the length of these tunnels - they go on for as far as the light can illuminate, squeezed between the seemingly endless corridor of rock in subtle tones of grey. It takes a while to get used to, but the absence of colour becomes noticeable and feels alien.

With two curious scientists asking questions on the road trip down to the lab (which takes about 30 minutes), Kim becomes our humanised Wikipedia, narrating the story with running commentary on the construction of the facility, the collaboration with the gold mine, and random little bits of fun facts about working deep below the surface. We pass several loud air ventilation systems and come across a couple of refuge chambers - sealed safety pods that supply oxygen, water, and shelter should an emergency occur. Nothing is left to chance in this hostile environment. 

Nearly a kilometre below the surface, construction on the SUPL began in 2019 after funding for the facility was included in that year’s Australian federal budget. The laboratory itself is a large cavity at this depth, approximately 12m high and wide, which has been excavated into the ancient rock. Inside sits a clean room that will host a suite of experiments, along with a separate ‘ante room’ that allows incoming equipment and instruments to equalise in temperature, shedding the excess heat they acquire from the surface or the journey through the tunnel network, and be cleaned before entering the main hall.

Our arrival at the lab was heralded by two very large doors that Kim pulled open. The first experience we have, stepping out of the vehicle, is the viscously thick wall of humidity that hits you like you’ve just stepped out of an air-conditioned aircraft in the middle of the tropics. The temperature down here is 35 degrees Celsius, generated by the heat from the rocks down this far below the surface (the rocky walls themselves are warm when I place my hand against them). The sweat room suddenly makes sense: without it, the heat obtained on the journey down would condense and collect in the main clean room.

The second thing is the dust. The moment I take one step onto the rock, I can see the small particles of grey dust sticking to the base and sides of my boots. For a moment, I imagine what it would have been like for the Moon walkers when they took their first steps on our neighbouring world, with the lunar regolith quickly clinging to their spacesuits. It’s for this reason that when we first enter the lab, we place plastic shoes atop our boots to ensure the facility remains as clean as possible. However, at this point, the air conditioning has helped relieve the stickiness from the outside environment. Kim walks us through a series of doors and hallways, work stations, kitchens, and side bathrooms before we enter the main chamber.

It's remarkable how clinical this enormous space feels - a complete contrast to the outside tunnels.  The laboratory itself is now complete and established - the walls are covered in a special type of painted material that assists with the radiation shielding, the crane is in place, a seismometer is set up in a side room, ventilation ducts and piping run around the perimeter, and there are even several muon detectors currently up and running.

Given the investment in the underground lab, SUPL has been developed as a multi-purpose research facility, hosting more than just the one dark matter experiment, and including experiments that are not related to geology and biology.

“SUPL provides a haven for groundbreaking precision measurement studies that could revolutionise scientific disciplines from physics to medicine,” said Kim.

“Shielded from cosmic radiation, SUPL provides researchers with the most controlled conditions available on Earth, enabling breakthrough discoveries that simply aren’t possible above ground. This is particularly important in fields like environmental monitoring, ultra-low background material screening, and medical research.” 

“Underground laboratories create opportunities for cross-disciplinary collaborations, bringing together environmental scientists, physicists, medical researchers, data scientists and others to solve complex problems from new angles.”

For the most part, our everyday interactions with the Universe around us consist of various wavelengths of light (radio, microwaves, infrared, optical, and UV) and baryonic matter. That is, the things that make up you, me, our homes, our families, the fridge, aircraft, buildings, cities, stars, galaxies, etc. -- they are all made up by the sub-atomic particles that we know about (as described by the standard model of particles: atoms, protons, electrons, neutrons, quarks, etc.).

Incredibly, this baryonic matter only accounts for a small portion of the entire matter in the Universe (roughly about 5%). There is another 25% or so, which is made up of something known as ‘dark matter’ - an invisible, yet-to-be-discovered form of matter. It has so far eluded discovery because it does not interact with the normal baryonic matter we have around us, nor with the radiation that spans the electromagnetic spectrum. We literally can’t see, probe, interact or directly measure it.

“In physics, it’s often the case that we discover things without ever directly seeing them,” said Katie. “In the same way you might understand that the wind is blowing by observing its consequences (trees shaking, leaves blowing past, the feeling of cold on your skin), we infer the existence of dark matter by how stars, galaxies, and even the shape of space itself respond to its presence. We see a huge number of different cosmic phenomena that can all be explained if there’s some kind of invisible matter that is providing extra gravity in galaxies and clusters of galaxies.”

“We don’t know what dark matter is made of, but based on our observations, it appears to be some kind of particle that has mass (it exerts gravitational pull) but doesn’t emit or absorb light and doesn’t seem to bounce off other particles when it encounters them,” said Katie with regards to why it has proven to be very challenging to detect this elusive material. “That makes it hard to see and hard to capture.”

For decades, astronomers have chased its shadow. Current models suggest that there are several different candidates for dark matter. This includes weakly interacting massive particles (WIMPs); axion-like fuzzy dark matter, and/or primordial black holes. 

Over the years, swarms of scientists from across the globe have dedicated their entire careers to searching for any hints, inference or direct detections of these candidates, with no avail so far. The time spent has not been wasteful, though, with some theories and candidates ruled out, and upper limits placed and constraints placed on others.

The hunt for dark matter in Australia has its roots in a mountain halfway across the world. Buried deep below the Gran Sasso massif in the Abruzzo region of Italy, lies the largest underground research facility in the world. Within it, it hosts an experiment designed to find the elusive dark matter signal, called DAMA/LIBRA. This experiment is a particle detector that exists in an extremely low-background radiation setting (within the mountain). 

The setup is both elegant and painstaking. The schematics include a central canister that features 25 thallium-doped sodium iodide (NaI(TI)) crystals arranged in a 5 x 5 matrix. Attached to the crystals are photomultipliers, designed to catch the tiniest burst of light and the entire assembly is contained within a sealed copper vessel filled with pure nitrogen. For additional protection against the elusive background radiation, the copper vessel is also cocooned and encased within 1m of concrete. It too is situated within a room where the bulk of the mountain aims to protect it against any background radiation. 

The principle of the experiment is simple, at least in theory. The idea is that it is searching for dark matter that is streaming through our Solar System, a feature of the dark matter that surrounds our Milky Way Galaxy. As our Sun (and thus the planets) traverse on their journey around the Galactic centre, a dark matter wind blows through our system, streaming from the direction the Sun travels towards. However, the Earth (and other bodies) all orbit the Sun, so at times of the year we are moving face-on into the stream, whilst at other times, this dark matter wind blows from behind us.

As the theory goes, every now and then, a single dark matter particle (in particular, a WIMP) might collide with one of these crystals and trigger a small flash of light that could effectively be detected. If confirmed, it would be the first firm indirect pieces of evidence of dark matter. Due to the motion of the Earth around the Sun, with the dark matter wind blowing towards us or from behind us, the number of interaction events should also modulate based on the time of year.

“Dark matter particles in the Galactic halo form a steady “dark matter wind”, said Profesor Elisabetta Barberio, from the University of Melbourne. Barberio is also currently serving s the Director of the ARC centre of Excellence for Dark Matter Particle Physics (CDM) .

“As Earth orbits the Sun, its speed through the wind changes over the year. This should cause a yearly rise and fall in the rate if nuclear-recil events in a detector - peaking around June and dipping around December”

Data collection at DAMA/LIBRA began in 2003, and since then, a suite of tantalising results have been released, with the exciting possibility that the dark matter signal has been detected. What has been presented is an annual repeating signal, rising and falling as one would expect from the annual change of Earth orbiting the Sun.

But not everyone is convinced, and these results are not without contention, as other, similar experiments in Spain (ANAIS-112) and South Korea (COSINE-100) have failed to reproduce these results.

Here is where Australia’s experiment steps in. As SUPL is located in the southern hemisphere, it is expected to also detect the same signal that has been detected by the northern hemisphere experiment, with a peak in June. However, if the origin of the signal is due to a terrestrial cause that is seasonal, then it could peak in December. 

“A true dark-matter signal should peak at the same time globally, because Earth’s velocity through the halo is the same regardless of hemisphere,” said Elisabetta. 

“But seasonal environmental backgrounds (temperature, radon, muons) peak six months apart between hemispheres. So, if SUPL sees a signal shifted by ~6 months relative to northern labs, it likely comes from seasonal background, not dark matter.”

Even then, researchers are tasked with a huge amount of work that is required to ensure the signal is, in fact, real and isolated from any background radiation or terrestrial effects. Everything - from something as small as humidity control in the experiment, to something as vague as a sudden influx of tourists in Stawell on the day the signal is detected, will need to be isolated and removed. As with all science, everything must be measured and accounted for when seeking the absolute truth. To do so, Elisabetta lists a suite of tools that the researchers will use.

“Deep underground location to reduce cosmic rays; heavy passive shielding plus active veto detectors; constant monitoring of temperature, radon, muon and electronics; detailed modelling of time-varying backgrounds, and multi-year data sets that will be cross-checked between hemispheres,” she said.

With knowledge from the learnings of the DAMA/LIBRA experiment in the northern hemisphere, researchers have now built an improved version called SABRE (Sodium-iodide with Active Background REjection) for SUPL. Lessons from the DAMA/LIBRA experiment in the north are going to be applied to the SABRE South experiment.

“Key improvements of SABRE South over DAMA/LIBRA include the ultra-pure NAI(TI) crystals with very low radioactivity similar or better than DAMA, surrounding liquid-scintillator veot and external muon veto, and lower expected background with better calibration and control of systematics,” said Elisabetta.

Once fully operational, the experiment will collect data continuously for years, slowly building a picture of what might - or might not - be there. “Commissioning is expected in 2026, with full science data-taking start by the end of 2026,” she said.

The findings from SABRE-SOUTH at SUPL are going to be pivotal on the global stage in the hunt for Dark Matter, whichever way the experiment results go. The unique location of SUPL in the southern hemisphere presents an opportunity to test the annual modulation of the signal, unlike the other experiments, which are based in the northern hemisphere. If it detects the same annual modulation as DAMA/LIBRA, it could mark the most compelling evidence yet for dark matter. 

Will dark matter remain as mysterious as it has ever been, or will we feeble humans, orbiting a generic star in a city of hundreds of billions of stars finally unlock one of the greatest mysteries we know about?

“It’s fair to say that all dark matter detection experiments are long shots,” Katie said, “but the payoff if we see something would be undeniably huge. If SABRE were to see an annual modulation signal, it would be an extremely intriguing result. No single experimental result is a smoking gun, but this would be something that, if confirmed with other experiments, would give us a way to learn what dark matter is really made of, and how it moves through our Galaxy.”

“On the other hand,” she continued, “If we don’t see a signal at all, it tells us that we need to look at different possibilities for what dark matter might be. The most interesting non-dark-matter result would be if we saw an annual modulation with the wrong phase, because that would tell us something about how our detectors work that could be used to improve other experiments.”

And so, deep beneath the quiet goldfields of regional Victoria, scientists prepare to listen for whispers from the cosmos - faint, yet important, signals that could rewrite our understanding of the Universe. Whether dark matter reveals itself or remains hidden, the search itself is putting Australian ingenuity and curiosity on the map and reminding us that even when we reach the edge of our collectively shared knowledge, we keep digging.

SpaceAustralia.com would like to sincerely thank the following individuals for their support in organising and facilitating the visit to the Stawell Underground Physics Laboratory:

Jackie Bondell, OzGrav / ARC Centre of Excellence for Dark Matter Particle Physics and Swinburne University

Kim Mintern-Lane, Stawell Underground Physics Laboratory

Fleur Morrison, OzGrav / University of Melbourne

We also extend our thanks to the wider OzGrav collaboration, ARC Centre of Excellence for Dark Matter Particle Physics, Stawell Underground Physics Laboratory, and the Stawell Gold mine for facilitating this visit.

SpaceAustralia ackowledges that the SUPL facility is built on the lands of the Wotjobaluk, Jaadwa, Jadawarjali, Wergaia, and Jupagulk people.