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MOST of our universe is missing. Observations of the smallest galaxies to structures spanning the entire universe show that ordinary matter 鈥� the stuff that makes up you, me and everything we see in the cosmos around us 鈥� accounts for only one-fifth of all matter. The remaining 80 per cent is a mystery.

After decades trying to hunt down this 鈥�dark matter鈥� with a series of ever-more-sophisticated detectors, we remain empty-handed. It is time for a new approach. Perhaps a combination of ancient minerals snatched from the bowels of the planet and modern nanotechnology will reveal the secrets of this elusive substance.

We cannot see dark matter because it doesn鈥檛 interact with light. However, theoretical models provide clues about what it might be made of. The most compelling explanation is that it is made of particles, just like all the ordinary stuff around us. One of the most promising candidates is the WIMP, or weakly interacting massive particle, a hypothetical entity with a mass similar to that of a normal atomic nucleus.

If dark matter is made of WIMPs, then a litre of stuff 鈥� whether it is air, rock or seemingly nothing at all 鈥� should contain a few of these exotic particles, whizzing around at speeds of hundreds of kilometres per second.

Most of the time, these particles would fly right through anything in their way as if it weren鈥檛 there. But very rarely, they could bump into the atomic nucleus of this matter and give it a little kick.

For the past 30 years, a number of experiments have tried to detect these collisions. This direct detection approach involves monitoring a lot of atomic nuclei in a laboratory to see if any get nudged by a dark matter particle. The largest such experiment, XENON1T at the Gran Sasso National Laboratory in Italy, used about 1 tonne of liquid xenon. The observations there lasted from late 2016 to early 2018, nuclei聽seems to have been .

Although dark matter hunters hoped that XENON1T or similar experiments would have seen something by now, this doesn鈥檛 mean the idea that dark matter is made of WIMPs is off the table. It is difficult to predict how often WIMPs should hit a nucleus 鈥� maybe we just need to watch 10 or 100 times more nuclei to see a collision.

A new way

Experiments with larger and more sensitive detectors aim to do exactly that. For example, the group that operated XENON1T is building a bigger version called XENONnT, using about 7 tonnes of liquid xenon. Of course, the more massive the detectors become, the more expensive they are to build and the harder they are to operate.

So some colleagues and I started thinking about alternatives. Rather than monitoring nuclei in real time, what if collisions could instead be recorded over an extremely long period of time? The material we need to do this already exists 鈥� it lies right under our feet.

Earth is around 4.5 billion years old. If we could find a substance that has 鈥渕emorised鈥� dark matter collisions for 1 billion years, for instance, then studying 100 grams of that material would be as effective as a direct-detection experiment watching 10,000 tonnes of nuclei for 10 years.

Our planet is full of ancient minerals with crystal structures, such as salts and diamond. In many such crystals, a collision between a nucleus and dark matter would knock the nucleus out of position (see 鈥�Diagram鈥�). Our calculations show that as the nucleus travels through the crystal, the trail of damage it leaves behind would be extremely short, typically less than 100 nanometres, or about 1000 times thinner than a human hair. In many materials, however, these tracks would stay imprinted for longer than a billion years. Thanks to modern nanotechnology, the tools now exist to reveal such tiny, fossilised features. Since this system uses ancient minerals, we called it a ().

How would palaeo-detectors work in practice? As with direct-detection experiments, false signals could complicate matters. Dark matter isn鈥檛 the only thing that can kick atomic nuclei out of position and in the 30 years of direct-detection experiments, physicists have gone to ever greater lengths to avoid this problem.

Cosmic rays are one major source of this noise. These ordinary, albeit highly energetic, particles constantly rain down on Earth from space. Like dark matter particles, they can nudge atomic nuclei when they pass through a material. However, cosmic rays cannot penetrate far into the planet, so direct-detection experiments are located deep underground. XENON1T, for example, sits 1.4 kilometres beneath Gran Sasso mountain. This is why the material needed for palaeo-detectors must be sourced from deep down too.

One benefit of our method is that it would only need a few kilograms of crystals. Such a modest amount could be sourced without too much difficulty from much deeper than direct-detection laboratories, which need to be accessible by people and trucks. This extra depth would give additional protection from cosmic rays.

Radioactive processes are another source of fake signals. Any material dug out of Earth is contaminated to some degree by radioactive elements such as uranium. Radioactivity causes damage features that could appear similar to those caused by dark matter. For example, radioactive processes produce energetic neutrons, which lose their energy by colliding with atomic nuclei and kicking them around, much like WIMPs could do.

To limit such noise, the cleanest possible materials must be used for palaeo-detection. Crystals formed in Earth鈥檚 crust are far too dirty, but nature has provided two much less contaminated sources: the planet鈥檚 seas and its mantle, the bit between the crust and core.

Minerals formed when magma cools underground, say, or formed at the bottom of an evaporating ocean are much cleaner than those formed in Earth鈥檚 crust because they tend to contain less uranium. Some of the minerals we have in mind for palaeo-detection are rock salt, epsomite and nchwaningite. These can be obtained from deep boreholes drilled for geological research or oil exploration. However, traces of radioactivity will be present in any material, setting a lower limit on the sensitivity of palaeo-detection.

鈥淔or decades, dark matter has escaped attempts to reveal its nature鈥�

Once a sufficiently pure ancient crystal sample has been obtained, the remaining task is to search for minuscule damage features left within it. These can be measured thanks to a range of sophisticated technologies developed in recent years, such as X-ray and helium-ion beam microscopy, which are powerful enough to zoom down to the nanometre scale.

It will be some time before we can search for dark matter with palaeo-detectors. So far, we have , showing what could be possible. Later this year, we will begin small, experimental tests. To ensure we can measure any damage tracks caused by WIMPs, we will create similar tracks in crystals using blasts of neutrons. We will also measure the radioactive contamination in a range of minerals from different places to see which might be the most promising for palaeo-detection.

For decades, dark matter has escaped attempts to reveal its nature. It could be that a combination of ancient minerals and modern technology might finally solve the mystery.