Dark matter

What’s invisible, but bigger than anything we can see? It sounds like the beginning of a bad joke, and for physicists, that’s just what it is.

All our cosmological observations seem to suggest that normal matter we can see – the sort that makes up you, me, the planet we live on, the sun our planet orbits and the galaxy we all exist in – is far outweighed by mysterious matter we can’t. Yet despite researchers’ best efforts over decades to work out the nature of this “dark matter” – to find some clue direct or indirect as to what it’s made of, or even make it in the lab – we remain stumped.

The story of dark matter begins in the 1930s, when the Swiss astronomer Fritz Zwicky noticed that galaxies towards the edge of the Coma cluster of galaxies were rotating faster around the cluster’s centre than they should have been, given the amount of visible matter there was there. But the concept really started gaining traction with the work of the US astronomer Vera Rubin from the 1970s onwards, who showed a consistent effect of spiral galaxies rotating too fast for the amount of visible matter present. Without more gravitating stuff to hold them together, these galaxies should simply fly apart. Our latest calculations suggest that dark matter makes up over 80 per cent of all matter out there.

Along with dark energy – another, even more mysterious substance that seems to have some kind of “anti-gravity” effect, causing the universe’s expansion to accelerate – dark matter represents a major embarrassment for physicists. Together, these two unknowns mean that over 95 per cent of the universe is made up of stuff not categorised in the standard model of particle physics, our best theoretical picture of material reality.

Not that we’re lacking in suggestions as to what might explain dark matter. Perhaps the most prevalent is that it is made up of an entirely new class of particles known as weakly interacting massive particles, or WIMPS, an invisible sea of which pervades galaxies. But if these particles exist, we should be able to make them by smashing together other particles, for example at the Large Hadron Collider near Geneva, Switzerland. You’d also expect these particles to be streaming through Earth – yet dedicated dark matter detectors, generally buried deep underground to screen out the effect of other cosmic particles, have not spotted them yet.

Another possibility is that dark matter is not WIMPs, but MACHOs. These Massive Astrophysical Compact Halo Objects are large bodies made of ordinary matter put together in unexpected ways that mean they don’t give off light. Very faint stars such as brown dwarfs, black holes and neutron stars have all been fingered as culprits, but it seems unlikely there are enough of these to account fully for the dark matter phenomenon.

A final, and controversial, suggestion is that dark matter doesn’t exist at all, and we’ve simply got our theories of gravity all wrong. But Einstein’s general theory of relativity, which governs gravity’s workings, is so good that this is a possibility few physicists are willing to countenance. Richard Webb