How do you find something that's invisible?

Dark Matter Iceberg Concept
(Credit: STFC/Ben Gilliland)

Call it Nature’s perfect practical joke. Call it the ultimate riddle posed by particle physics. Whichever way you look at it – if you could actually look at it – so-called ‘dark matter’ is a real cosmic enigma. Cloaked in invisibility because it stubbornly refuses to emit or reflect any light, not only are we humans unable to see it but, to date, no instrument or experiment ever developed has seen it either. Yet dark matter accounts for an estimated 27% of the Universe, dwarfing the mere 5% accounted for by ‘regular’ matter.

All of which presents both a frustrating problem and a fascinating challenge. Until we know exactly what dark matter consists of and precisely how it behaves, we can’t fully understand the Universe, its underlying structure and its critical workings. Not surprisingly, then, the pursuit of this shadowy substance has become a key goal of particle physics.

So how can we be confident that dark matter really exists? Although it’s invisible, it betrays its presence by pulling, tugging and influencing things we can see. For instance, it affects how our Solar System moves around the centre of the Milky Way and, in terms of the larger-scale structure of the Universe, helps to shape clusters and super-clusters of galaxies. Dark matter also bends light reaching across space from galaxies billions of light years away.

Pinpointing the effects of dark matter only takes us so far. When it comes to unpicking dark matter’s darkest secrets, it’s ultimately no substitute for observing more directly the fundamental particles that make up this mysterious material. Or trying to. So far, all attempts to do so have ended in failure.

Dark matter: The matter we can't see - James Gillies
(Credit: Ted-Ed)

Enter the WIMPs. It has been suggested that Weakly Interacting Massive Particles, or WIMPs, are a prime suspect for being dark matter. If they exist, on the rare occasions they interact or collide with each other their annihilation would generate gamma rays, neutrinos or other high-energy particles that give away their existence. Looking for such tell-tale signs – a method known as indirect detection – is high on the agenda of a host of current initiatives worldwide looking for dark matter. For example, the multinational Cherenkov Telescope Array (CTA) project, which STFC is helping to fund, aims to develop unprecedented capability in the detection of gamma rays produced by WIMP annihilation.

But direct detection also offers intriguing possibilities. Here, the focus is on detecting the fleetingly rare interactions between WIMPs and atomic nuclei, and specifically the snooker-like effect whereby a collision causes the atomic nucleus to recoil. The UK is a big player in this field, established as a world leader in the development of detection technology, and contributing to key international projects. For instance, the DRIFT II collaboration at STFC’s Boulby Underground Laboratory – the UK’s remarkable deep underground science facility located on the edge of the North Yorkshire Moors – is the world’s most sensitive detector of its type, designed to detect the tracks of the atomic nuclei that WIMPs collide with. Siting it underground shields the detector from cosmic rays that could be confused with the signals a WIMP leaves behind.

The UK is also heavily involved in the US-led LUX-ZEPLIN collaboration, set to deliver dark matter detection capabilities over 100 times more sensitive than any previous experiment of its kind. Here, the centrepiece will be a 7-tonne liquid xenon ‘target’. If a WIMP arrives from space and strikes a xenon nucleus that then recoils through the liquid xenon, the result will be a flash of light and an electrical signal – confirming the WIMP’s arrival.

But perhaps the most enthralling possibility of all is that we could actually make our own dark matter right here on Earth. Predictably, the Large Hadron Collider (LHC) – the world’s biggest particle accelerator, housed at the European Organization for Nuclear Research (CERN) in Switzerland – is setting the pace. The plan is that, following a two-year upgrade now coming to an end, the LHC will deliver proton collisions of unprecedented energy. If the motion of the visible matter created by these collisions can only be explained by the production of new invisible particles, it could provide the more direct evidence for dark matter that science has held its breath for. Indeed, the new, higher-energy LHC could be shown to be acting as a ‘dark matter factory’ where protons are routinely transformed into dark matter particles. That, of course, would mean the LHC is actually increasing the quantity of dark matter in the Universe – by a minuscule amount, certainly, but still quite an incredible thought.

Whatever the future holds, creating dark matter or observing it more directly is unquestionably one of the biggest prizes on offer in particle physics today. Fittingly, seizing that prize would leave science a little less in the dark about the Universe, its true nature – and its ultimate destiny.

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