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The universe doesn鈥檛 make sense without dark matter. According to our best theories, this invisible substance acts as a source of extra gravity in the cosmos, which is needed to keep galaxies from spinning themselves apart. Astronomers see its ghostly influence in everything from the bending of the light of distant stars to ripples in the big bang鈥檚 afterglow.

But what is it? It鈥檚 a tough question to answer given that we can鈥檛 see it and can hardly touch it. With little hard evidence to go on, however, there鈥檚 no shortage of theories.

WIMPs

There are two basic requirement for any dark-matter particle. First, it must have mass, so that it feels and generates gravity. Second, it must have no electric charge聽鈥� so it does not interact with light, making it invisible. Step forward the WIMP: the weakly interacting massive particle.

A thick, slow-moving soup of WIMPs would be just the thing to explain the distribution of galaxies across the cosmos. As it clumped together, galaxies would have formed with the pattern of clusters and chains we see today. Better still, the number of WIMPs created in the big bang neatly fits our estimates of dark matter density聽鈥� a coincidence known as the WIMP miracle.

There鈥檚 just one problem: a gaggle of detectors built to pick up signals from the rare occasions when WIMPs collide with atomic nuclei have ruled out the particles鈥� existence, at least in the mass range where we hoped to find them. That includes results last month from the Large Underground Xenon (LUX) detector, one of the largest.

The book isn鈥檛 closed yet, but it seems that if WIMPs exist, they must be lighter than we thought聽鈥� perhaps too light to explain the structure of the cosmos.

MACHOs

If WIMPs aren鈥檛 to your taste, try MACHOs. The idea here is that rather than being made of unknown particles, dark matter is just normal stuff hiding at the edges of galaxies聽鈥� 鈥渕assive astrophysical compact halo objects鈥� that are sufficiently dim to be all but invisible to our telescopes. Candidates include black holes or failed stars called brown dwarfs, warm balls of gas that don鈥檛 have enough mass to fire up nuclear fusion.

The way to unmask a MACHO would be to see how it bends light as it passes in front of a visible star. Some researchers claim to have seen such gravitational lensing effects, but it鈥檚 also become clear that MACHOs could only account for a tiny fraction of the universe鈥檚 missing mass.

Macros

Running with the idea that dark matter is just normal matter in disguise, perhaps it is much smaller and weirder than a MACHO. Macros are dense clumps of quarks, the fundamental particles that form ordinary matter when in pairs or triplets. Although tiny, macros would be as dense as neutron stars and extremely heavy, with masses measured in kilograms. Unfortunately, the experiments needed to spot them, such as seismometers on the moon, are pretty outlandish, so this idea is likely to remain in the realms of theory.

Axions

Originally invented to fix a hole in the theory of the strong nuclear force, which keeps atomic nuclei together, axions have since proved useful elsewhere聽鈥� and they might just solve the dark matter problem in their spare time. Like WIMPs, these particles have no charge and barely rub shoulders with ordinary matter. They are even punier, meaning they would interact more tenuously with familiar stuff.

That also suggests WIMP detectors could have spotted them already聽鈥� but they haven鈥檛. The jury is still out, at least until dedicated experiments such as the Axion Dark Matter Experiment (ADMX) at the University of Washington in Seattle return a verdict.

Sterile neutrinos

If dark matter is formed of particles that have a bit of mass but hardly interact with anything, surely we already have prime candidates: neutrinos. These particles pass through other matter almost as if it doesn鈥檛 exist, and come in three varieties聽鈥� all of which turn out to be too light and zippy for dark matter. What we need is a heavier, more aloof version: the 鈥渟terile鈥� neutrino.

Signs of sterile neutrinos have emerged in underground detectors, only to quickly disappear. An excess X-ray signal seen in 2014 from galaxy clusters expected to be rich in dark matter was also thought to be their signature聽鈥� but in the end, the source couldn鈥檛 be pinned down.

Gravitinos

If sterile neutrinos seem too mainstream, how about the gravitino? This is the hypothetical 鈥渟uperpartner鈥� of the graviton, itself a hypothetical particle proposed to mediate the force of gravity. The gravitino pops out of the theory of supersymmetry, a long-favoured extension to today鈥檚 standard model of particle physics, and it has many of the characteristics expected of a dark-matter particle. The trouble is that supersymmetry鈥檚 star is on the wane聽鈥� the theory predicts heavier partner particles for all our familiar ones, but despite our best efforts, machines such as CERN鈥檚 Large Hadron Collider have provided no sign of them.

MOND

Once we鈥檝e exhausted all the particle guises dark matter might take, we might have to countenance another possibility: it鈥檚 not there at all. That鈥檚 the rationale behind the maverick theory of modified Newtonian dynamics (MOND), which says we don鈥檛 need dark matter if we tweak the laws of gravity laid down by Newton and Einstein.

The very thought makes many physicists uncomfortable, but there have been hints that it might work. And last year, Justin Khoury at the University of Pennsylvania proposed a hybrid model that reconciles MOND鈥檚 success at galactic scales with the success of WIMP-like models on even larger cosmological scales聽鈥� a superfluid form of dark matter that acts like WIMPs in individual galaxies, but modifies gravity on bigger scales.