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IF YOU happen to pass through Antarctica later this year, you could be greeted with a peculiar sight. Peel your eyes away from the penguins and you might spot something unusual floating in the sky: a balloon the size of a stadium. Trailing below it will be the latest mad-sounding experiment designed to look for the most maddening thing scientists have ever dreamed up – dark matter.

We reckon around 85 per cent of the universe’s matter is exotic stuff that doesn’t reflect, emit or absorb light, which is why it is called dark matter. The only force that this hypothetical stuff definitely deigns to interact with is gravity, as far as we know, which makes it incredibly difficult to detect. “When I gave talks on this in the 80s, I was telling people, ‘Oh, we’re going to figure this out in 10 years’,” says , an astrophysicist at the University of Texas at Austin. Decades later, we are still waiting. “It’s obviously a harder problem than we realised.”

In the face of that hard truth, dark matter hunters have become ever more inventive. Attempts over the years to pin down what it is made of include burying vats of liquid xenon deep underground, measuring the straightness of lightning bolts, a plan to detect nanoscale explosions in minerals, examining ancient rocks for dark matter scars and checking the James Webb Space Telescope’s observations for “dark stars”. All of which raises the question: are some suggestions for dark matter searches a long shot too far? And at what point would we consider giving up the chase?

The first hints that something dark permeated the universe came in the 1930s, when astronomers spotted something odd. Clusters of galaxies were rotating far faster than they should have been, for instance. At such rotation speeds, from our understanding of gravity and dynamics, some of their constituent parts should have been flung off into the depths of space, but they were holding together. Maybe, suggested Swiss astronomer Fritz Zwicky, there was some matter that we couldn’t see holding everything in place.

The hunt for Zwicky’s “dark material” never really took off, partly because there were lots of questionable assumptions behind his assertion. Things changed in 1970, though, when Vera Rubin and Kent Ford at the Carnegie Institution of Science in Washington DC made a surprising discovery about the Andromeda galaxy. They noticed a small-scale version of Zwicky’s observation: the stars within the galaxy were rotating around its centre faster than expected. They should have been flung off, but, again, something was holding them in place.

This is when the hunt for dark matter began. Rubin predicted we would know exactly what this strange galaxy glue was within a decade. 1980 came and went, however, and we were none the wiser. In 1999, English Astronomer Royal Martin Rees again predicted we would have found it within a decade. We didn’t. “My confidence in us quickly pinning down the nature of dark matter was certainly misplaced,” says Rees. Today, we still don’t know what dark matter is made of.

MACHOs and WIMPs

The decades since Rees’s prediction have helped us rule out a few potential candidates. Peering into the distant universe has led us to be pretty sure that it isn’t comprised of massive planets or black holes, known as massive compact halo objects, or MACHOs. If it were, we would probably have seen these objects bending cosmic light in predictable ways.

For much of the past 40 years, physicists have been on the search for another dark matter candidate: weakly interacting massive particles (WIMPs). These would be particles not found in the standard model of particle physics, our current best understanding of the forces and particles that make up the most fundamental building blocks of nature.

WIMPs were dreamed up with all the attributes dark matter seemed to exhibit. They also had the desirable virtue of being within reach of purpose-built detectors, should their trajectory happen to collide with an atomic nucleus. If a WIMP gave it a reasonable kick, the nucleus’s recoil energy would then be released as a flash of light that we could spot.

Supercooled xenon

Getting a detection requires having a lot of large nuclei. That led physicists to design and build detectors that use enormous vats of supercooled liquid xenon, ready and waiting for a kick from a WIMP. These kinds of experiments are still widely considered our best bet to find dark matter. The latest is the proposed for the Gran Sasso Laboratory in Italy, which will use 50 tonnes of xenon. Annual global production is around 70 tonnes; physicists really are going all out on this idea.

But there are no guarantees. For a start, most of the potential masses that WIMPs might come with have been ruled out. If WIMPs were on the heavy side, we would have seen them by now in underground vats or as a product of protons smashing together at the Large Hadron Collider near Geneva, Switzerland.

The tricky thing with WIMPs is that the lighter they are, the harder they are to find. Most current detection methods largely rely on the WIMP having a certain minimum mass, around that of 10 electrons. “[Detectors] are underground waiting for a dark matter particle to hit a xenon atom and make it wiggle,” says at Durham University, UK. “But if the particle is too light, the xenon atom is not going to recoil.” In that case, physicists run into another problem. Neutrinos produced by the sun are passing through Earth in their trillions every second. They would also leave traces in the xenon that are impossible to disentangle from those of light WIMPs. “There’s no way to shield against solar neutrinos,” says Bauer.

One alternative looks deep into the past. This is arguably the most impactful melding of archaeology with astronomy since we first made sense of Stonehenge. The idea is simple. WIMPs might, just occasionally, have knocked an atomic nucleus out of place in a crystal of rock salt or epsomite deep in Earth’s mantle. If this happened, relatively new technologies such as X-ray or helium-ion beam microscopy might be able to see these displacements as tiny, telltale tracks in ancient rocks.

This particular hunt for dark matter is starting to come together. “We’ve got geophysicists telling us which rocks to look for,” says Freese. “We did the calculations to figure out how much rock you would need to see a definitive trace from dark matter, and it’s not that much.” Once the rocks are extracted, the researchers performing the will have to act fast, because cosmic rays will then start to leave similar nanoscale tracks in their structure. But it should still be possible to distinguish these from those caused by WIMPs and those created by fission products released by naturally occurring uranium.

Freese, for one, is loving the hunt for dark matter, which she has been involved with for decades now, calculating how various kinds of particles might be discovered. “It’s rough that nothing’s been seen, but the calculations we did were just the easiest ones to find,” she says. “I’m not giving up. In fact, we’re really having fun.”

Some researchers get their kicks by chasing alternative hypothetical particles, such as the axion. Unlike WIMPs, axions weren’t cooked up as part of the search for dark matter. Instead, they were first proposed as an attempt to solve an anomaly where experimental data and an aspect of quantum theory don’t quite match up. But axions should also have the properties we associate with dark matter. And, in experimental terms, the search for them is pretty much a blank slate. The seeks to convert axions to microwave photons using a strong magnetic field, but other approaches, such as looking at the light from the sun or seeing axions’ effects on magnetometers, are .

Dark photons and dark stars

Still others are looking for a dark equivalent for each particle in the standard model. Searches for this “dark sector” have begun, with researchers firing high-intensity electron beams at targets in the hope that they might emit a “dark photon”, for instance. That would be massless, so it wouldn’t be dark matter exactly, but it would tell us the dark sector is real – and compel us to hunt other dark particles. “A lot of people are working on the dark sector,” says Freese. “Lots of things like dark photons are not that much of a stretch.”

But particles aren’t the only contenders. Freese is also looking for “dark stars” – balls of dark matter that . One hypothesis says that if dark matter particles interact, they annihilate. If dark matter interacted with itself just a little bit, this would release enough energy to create stars long before the ones we are familiar with started to shine. As Freese and her colleagues reported last year, instruments on the James Webb Space Telescope (JWST) might be able to show us these stars. Due to their distance from us in an expanding universe, their light would appear redder than less ancient stars. “[JWST is] already finding too many bright objects at high redshift. Once we get their spectra, we should be able to learn something about them,” she says. “We’ll be asking as the data comes in, could this be a dark star?”

Clearly, the possibilities are legion. For some, though, the fact that pretty much “anything goes” is a sign that the whole enterprise is a wild-goose chase (see “Dead ends?” below). “You can’t just keep moving the goalposts,” says Stacy McGaugh at Case Western Reserve University in Ohio. “There has to be a point at which you decide you’ve done what you can, and you stop.” McGaugh gave up on dark matter decades ago. Now, he is working on the idea that we can explain the galaxy rotation anomalies in a different way.

Redefining gravity

McGaugh is a supporter of what is called modified Newtonian dynamics (MOND) – a controversial idea that proposes rewriting the laws of gravity. One way to do this is by altering the equation of Newton’s law of universal gravitation in a way that changes how strong gravitational attraction between two masses is over cosmological scales. McGaugh claims that, in many cases, it provides a better fit to observational data than dark matter. Most importantly, he says, it has a predictive power. “You can look at a galaxy, observe its distribution of mass and you can use MOND to predict the way everything moves,” says McGaugh. “You can’t do that with dark matter.”

Most astrophysicists dismiss the idea. “They’ve never demonstrated that they can explain all the basic observations that we have,” says Kathryn Zurek at the California Institute of Technology. It falls short, she says, in explaining the features of the radiation left over from the big bang, known as the cosmic microwave background. “You just can’t get all of that from MOND.”

McGaugh admits that MOND doesn’t make sense of all our observations, but, he says, it has scored some successes over dark matter. For one, JWST spotted old galaxies that are brighter than they should be according to standard cosmological theory. In 1998, McGaugh points out, Robert Sanders at the University of Groningen in the Netherlands found that modified gravity would result in bright galaxies forming extremely early in the universe’s history – by 500 million years after the big bang. “That’s exactly what we’re seeing now,” says McGaugh.

Most in the field are a long way from being ready to give up on dark matter, though. We may have ruled out a few potentials, says Rees, but plenty remain. “The odds in favour of some as-yet-unspecified particles constituting the dark matter are not significantly reduced.”

Perhaps the Antarctic blimp will find antiprotons – the antimatter equivalent of protons – in the flux of cosmic rays that stream towards Earth from space. Models of dark sector dark matter in this particle stream, so any detections during the planned series of 35-day flights would be an exciting development.

Even if nothing turns up, it is important to note that the search for dark matter is a relatively small-scale enterprise. We spend far less on it than we do on high-energy physics or quantum computing research, says Zurek. “Plus, we’re learning stuff,” she says. “It’s taught people to not be quite so dogmatic about how nature should behave. It’s useful and humbling.”

No one can know when, or how, or even if, dark matter will show up in the end. But Zurek is OK with that. “I think we’re going to have to get lucky,” she says. “But if we don’t look, we’re certainly not going to find it.”