By Liam Mannix
To imagine dark matter, fill your mind’s eye with an image of the universe: a speckle of planets, stars, galaxies and black holes painted across a vast black tapestry.
Now subtract those things, one by one. No planets, no stars, no galaxies.
They are just distractions from the real universe. “The icing on the cake”, astronomers say.
It’s dark matter’s universe, we’re just living in it.Credit: KIPAC/SLAC National Accelerator Laboratory
You are now viewing 80 per cent of the universe’s matter. Everywhere - everywhere - you look, there is dark matter. This shadow universe stretches out like a spider’s web whose ribs act as the foundation for, well, everything.
Without dark matter, so the thinking goes, the universe as we know it would not exist at all.
Where better to imagine this shadow universe than in a cave? When they turn the lights off at the bottom of Stawell’s gold mine, the darkness is absolute.
A team of Australian scientists are down here, building a laboratory to hunt for dark matter.
Down into the bowels of the Earth.Credit: Justin McManus
They already have tantalising proof it exists. Their experiment will either confirm those findings - and put someone on a short path towards a Nobel Prize - or refute them, suggesting, perhaps, that there is something fundamental about the universe we simply do not understand.
An underground laboratory
The most unexpected thing about being 1025 metres below the surface of the Earth is that it is really hot and really humid. Arriving at the bottom of the mineshaft is like landing at an airport on a tropical island, except everything is black and dirty and you are required to carry your own emergency oxygen supply.
The shaft is narrow, steep and twisting. Our Toyota HiLux takes about 30 minutes to drive the eight kilometres to the bottom, where the shaft takes a sharp left and then suddenly levels and opens out. We get out and walk into a huge room carved out of the rock.
At the entrance to the dark matter laboratory.Credit: Joe Armao
Elisabetta Barberio runs her hand along the undulating walls. Painted white and bathed in floodlights, it has the quality of an underground cathedral. Our voices echo up to the 14-metre-high ceiling.
“We are at a point in physics,” she says, “where everything we think we know is only a tiny fraction of the universe.”
She is an explorer searching for a particle unseeable and untouchable.
What is dark matter?
The atoms that make up you and me and everything we see can interact with the world via four forces of nature. Dark matter only gets one: gravity. You can’t touch it. It does not reflect light. It is a ghost.
Its story begins a sliver of a second after the big bang.
The early universe was dense, hot and full of dark matter; as it expanded and cooled, the dark matter stretched out with it, like a ball of dough spread onto a counter.
Dark matter was first predicted in the 1930s, and strong evidence amassed in the ’60s, but it took until the ’90s for scientists to find a way to really “see” it. Over a long enough distance, light gets pulled around by gravity; scientists found they could trace that path as light is warped by the gravity of dark matter, and produce a map.
Everyone knew there was a lot of dark matter out there. But the first maps still came as a shock.
“The visible galaxy is just a little thing in the middle of this enormous dark body,” says Professor Kenneth Freeman, an Australian National University astronomer who has made several key contributions to the field.
“You don’t know what it is, but it is there. It changed our whole ideas about how galaxies formed, how the universe evolved.”
The big bang blasted out a physics-confetti of particles across the universe. When physicists simulate the big bang on supercomputers minus dark matter, the universe they get looks nothing like ours.
A map of the distribution of the universe’s dark matter. The bright areas represent the highest concentrations - they are also where galaxies form.
To turn chaos into stuff, something needs to give it shape. In our universe, the web of dark matter plays that role. Its gravity tugs the universe into shape. Gas becomes stars and dust becomes planets.
“Once you add in dark matter the sims are really astonishingly good at reproducing what we see,” says Professor Tamara Davis, a dark matter astrophysicist at the University of Queensland.
“The fact there is something out there that needs to be explained is without doubt.
“We have now measured many of its properties, but we still don’t know what it is.”
The power of WIMPs
For more than 50 years, scientists have been searching for it. Many candidates, like black holes, have been ruled out. The searchlight is now trained on WIMPs: Weakly Interacting Massive Particles.
These particles would be heavy and slow-moving, physicists think, and float through normal matter almost without trace, interacting with our world mainly via gravity. They are not made of the electrons and protons that make up our atoms but of something else altogether.
“It’s almost like this parallel universe of dark matter which sits on top of what we see in daily life,” says Professor Ulrik Egede, a particle physicist at Monash University.
So far no one has found one, not through years of search, not through smashing atoms harder and harder together at the Large Hadron Collider.
Well, almost no one. There is a lab buried under a mountain in Italy hearing a very curious signal. More on that in a moment.
Professor Elisabetta Barberio stands inside the underground lab.Credit: Joe Armao
Barberio’s underground cathedral took eight months to excavate, blasted out of the basalt in four-metre increments. “We had already drilled 200 kilometres of tunnels. Elisabetta wanted one of her own,” says Troy Cole, Stawell Gold Mines’ general manager.
Six-metre-long rock-bolts were drilled into the walls to hold them steady, and then everything covered by a layer of shotcrete and two layers of radiation-blocking TekFlex. There’s about as much engineering down here as the Burnley Tunnel, the mining engineers say. Next to come is a 10½-metre crane, to handle the 200-tonne iron-and-polyethylene shield that will cover the detector.
When she first came down here in 2014, Barberio remembers picking over cobwebs and past disused machinery. The mine was dying.
Stawell’s gold mining history stretches back to the 1850s, when tens of thousands of prospectors were drawn to the city’s goldfields, lured by dreams of a nugget to make their fortune.
About 250 kilometres of tunnel now wind under Stawell, but there are no nuggets left. Instead, piles of crushed ore are brought to the surface and then treated to extract a tiny amount of gold - perhaps four teaspoons in every truckload.
Trucks enter the mine. A full tray of rock produces about two teaspoons of gold.Credit: Joe Armao
“We were running out of ore,” says David Coe, health, safety, environment and community manager at Stawell Gold Mines. “We had got to 1646 metres in depth, and there was no more ore to chase. And we knew eventually we would close.”
In 2012, Stawell Gold Mines started searching for a future for the mine (one idea that thankfully never got up: turn the mine into an underground mushroom farm). Coincidentally, Swinburne astrophysicist Jeremy Mould had started a letter-writing campaign to the nation’s underground mines, looking for somewhere to house a small dark matter experiment. Separately, Professor Barberio - based at the University of Melbourne and now director of the new Centre for Dark Matter Particle Physics - was in talks with Italian colleagues about doing their own dark matter search.
The three groups found each other and in 2014 toured the mine site, discovering it was perfect. Planning for the lab started in earnest.
Then in 2016, the ground disappeared from under their feet. The mine’s owners put the site into caretaker mode. Cole, who had worked on the site since 1998, had to watch as the 250 employees shrank to just 12. The lab looked finished.
In 2017, Arete Capital Partners, a global private equity group, purchased the mine and restarted operations. Meanwhile, the scientists had pulled together a coalition of funders including the federal and state government and the Italian Istituto Nazionale di Fisica Nucleare. In 2019, with funding secure, excavation began; the hope is the lab will be up and running next year.
It is hoped it will operate for at least 50 years, drawing scientists and investment from around the world. There’s nothing else like it in the southern hemisphere. Full-time employment at the mine is now back up to 243.
“It’s definitely gone through a rollercoaster of emotion,” says Cole. “From a sector that was just about packed in, to where it is now, it’s good.”
Keeping the ‘noise’ down
Why go to all this effort to build a lab underground? Because to detect the impossibly faint signal of dark matter, one must first mute the deafening roar of the universe.
Radiation shows up as static on a dark matter detector. And everything emits radiation, even bananas. That’s part of the reason it’s so hot down here: the rock surrounding us contains tiny traces of uranium, which decay and release heat.
Take pouring the concrete floor of the lab. Barberio’s team spent six months testing cement samples for radiation levels, eventually settling on a special product from Brisbane. When researchers arrive at the lab they will have to shower, lest their bodies carry tiny radiation-emitting particles from the surface.
In the coming months, the crane will carefully bring in the dark matter detector itself, which looks like an oversized oil drum studded with thick cables.
Inside, sitting in a chemical bath, are an array of hollow copper tubes. Inside those tubes sit seven ultra-pure thallium-doped sodium iodide crystals - the purest ever made.
A cutaway of the dark matter detector. The copper tubes containing the crystals are at the centre.
Like dark matter, these crystals are also strange artefacts. Atoms in the crystals are packed so tightly together that, in theory, eventually a dark matter particle has to hit one. If it works, the lab thinks it might get three collisions a month. These collisions will release energy which can be picked up by a set of detectors surrounding the crystals.
“The crystal will see everything. If it sees dark matter, it will produce light,” says Barberio.
Which brings us to the lab in Italy, buried under a mountain.
For 20 years scientists working in a lab beneath Gran Sasso, a snow-covered peak in the Apennine range, say they have been detecting dark matter.
Almost no one believes them. A range of other experiments looking in the same range have found nothing. “There are many other experiments on Earth that are looking for dark matter, they have seen nothing, they have not seen a hint,” says Egede.
That said, no one has been able to offer a good explanation for what the Italians are seeing either.
Perhaps the most convincing part of the Italians’ signal is the way it changes. Imagine the planet Earth as it orbits the sun, as the sun itself moves through the web of invisible dark matter. In summer, the Earth’s orbital path takes it in the same direction as the sun, and in winter it takes it in the opposite direction. This is exactly what the Italians see: as Earth moves faster through the dark matter in summer, the signal gets stronger, and in winter it gets weaker.
That’s why the Stawell lab is being built, using a detector identical to the Italians’. If it finds the same signal, that would be “pretty strong” proof, says Egede, time to start booking your tickets to Stockholm.
What happens if they don’t find a signal is almost more intriguing. Because we are fast reaching the point where detectors are sensitive enough that there will be no place for dark matter to hide.
Science works by putting up a theory and then testing it until it breaks. In perhaps 20 years’ time, we’ll be at that point with dark matter. “If we still haven’t seen dark matter, it is hard to see how we could,” says Egede.
That opens a second possibility: there’s something bending light and exerting enormous gravitational pull on galaxies that we don’t understand. Some new physics.
“The concept [that] our law of gravity might be wrong,” says the University of Queensland’s Professor Tamara Davis, “remains a live proposition.”
Barberio is not worried. “More and more,” she says, “we think dark matter is more complex than we thought at the beginning.”
“More and more,” says Professor Elisabetta Barberio, “we think dark matter is more complex than we thought at the beginning.”Credit: Joe Armao