September 2, 2026 — 12:01amExamine, a free newsletter covering science, is sent every Tuesday. You’re reading an excerpt – sign up to get the whole newsletter in your inbox.Sometimes it feels as if all the fundamental questions about existence have been solved. Then you remember dark matter.This unknowable substance constitutes as much as 85 per cent of all mass in the universe, and yet we cannot see or directly measure it, and we don’t know what it’s made of.Scientists believe dark matter underpins the structure of the universe.KIPAC/SLAC National Accelerator LaboratoryBut the structure of spiralling galaxies, the make-up of the cosmos, and indeed our very existence only make sense if the universe is woven with an undiscovered, invisible essence, holding it all together like a secret skeleton.Needless to say, dark matter is the greatest scientific mystery of our time.That’s why physicists are intensely excited about a tiny spark of light detected in a pool of liquid xenon in the depths of an abandoned gold mine.The data is limited and fresh, and scientists are duly cautious. But there is a possibility this newly reported signal could be the first-ever detection of dark matter.How do we know dark matter exists?Fritz Zwicky was an unconventional and at times unpopular scientist who once said of his more orthodox colleagues: “Astronomers are spherical bastards. No matter how you look at them, they are just bastards.”The Swiss stargazer was investigating the Coma galaxy cluster in 1933 when he realised the galaxies were rocketing around each other at 2000 kilometres per second – so fast they should have been violently flung away from each other.The structure and speed of galaxies don’t make sense unless they contain far more mass and gravity than they appear to have.APHe calculated the galaxies, to summon enough gravity to stay together, must have 400 times more mass than they appeared to have. What was providing this invisible gravitational glue? He proposed the existence of dark matter.Zwicky’s theory was treated as zany. Then American astronomer Vera Rubin spied another anomaly in the 1970s.Stars on the outer edge of a galaxy should orbit much more slowly than those at the centre. But Rubin discovered the outer stars of a galaxy travelled just as fast as those closer to the centre.The visible matter in the galaxies didn’t have enough gravity to hold on to these distant, fast-spinning stars. Something else was holding on to them. Rubin concluded: “What you see in a spiral galaxy is not what you get”, and that dark matter must be real.Now astronomers can see how light is warped in space by gravity emanating from dense, invisible regions of suspected dark matter.The image below shows galactic gas highlighted in pink, while the blue areas show regions of intense gravity scientists believe can only be explained by dark matter.The Bullet Cluster, a pair of galaxy clusters that collided. Normal matter is shown in pink, suspected dark matter in the cluster shown in blue. This observation has provided one of the clearest direct examples of dark matter.NASABut that’s a shadow of a shadow. In the race to truly detect dark matter, instead of looking up through telescopes, we’re burrowing deep into the Earth.The deep detectorDark matter doesn’t emit or reflect light, and it barely interacts with normal matter. We also don’t know what it’s made of. So how do we find it?Dr Theresa Fruth, a dark matter hunter from the University of Sydney, helped design and construct one of the best dark matter detectors on Earth, called the LUX-ZEPLIN (LZ) experiment. About 250 scientists and engineers are participating in the project.Dr Theresa Fruth, from the University of Sydney, preparing to descend to the LZ detector in South Dakota.Theresa Fruth“It’s basically a big bucket of liquid xenon,” Fruth says. “Xenon is a trace element in the air around us, and it’s a noble gas. But if you cool it down enough, about minus 100 degrees, it’s a really dense liquid.”Any skerrick of radiation might mimic dark matter and must be banished.“Xenon is good because you don’t have any long-lived radioactivity naturally in the xenon. It decays away and you can have this very clean, very quiet liquid.”The LZ detector was moved 1.5 kilometres underground into a gold mine to shield it from cosmic rays.Matthew Kapust/Sanford Underground Research FacilityResearchers assess LZ’s central detector while it is wrapped in foil.Matthew Kapust/Sanford Underground Research Laboratory