THERE鈥檚 something out there, astronomers say, that makes up 90 per cent of the Universe but鈥ell, unfortunately, we can鈥檛 see it and we don鈥檛 know what it is.
Should we doubt what astronomers are telling us? At times, dark matter does smell suspiciously of those other invisible substances, phlogiston and the ether, that scientists dreamed up as physical fig leafs to make sense of otherwise inexplicable observations. They turned out to be phantoms, and so indeed might dark matter.
Physicists invented it some 30 years ago to explain the rapid spin of certain galaxies. The argument went that some kind of dark matter, invisible to astronomers鈥� telescopes, must be providing the extra gravitational glue to stop these galaxies flying apart. The idea that most matter is dark matter later turned out to fit with other observations, notably the large-scale structure of the Universe.
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But despite prolonged searches, no laboratory experiment has yet detected any dark matter. A hint of a sighting at the DAMA project in Gran Sasso, Italy, has been treated with scepticism by most of the 20 or more rival labs. And there is another reason why dark matter is suffering a bit of a slump. Physicists鈥� favourite candidate for what the stuff is made of, called weakly interacting massive particles or WIMPs, does not fit with all the astronomers鈥� observations (see 鈥淭he new dark age鈥�). So scientists have had to go back to the drawing board to create a set of new hypotheses about the kinds of peculiar particles that could make up dark matter. Right now it is not looking like a very tidy theory.
All this leaves the small opposing camp cheering loudly. The supporters of modified Newtonian dynamics (MOND) claim that cold dark matter doesn鈥檛 exist. Instead of inventing new forms of matter, they say, we must adjust our equations of gravity to make its attraction fade away more slowly with distance (see 鈥淏reaking the law鈥�). Then it would be strong enough on the outskirts of galaxies to hold them together.
So is it to be new matter or new gravity? In the absence of firm evidence for either side, it remains a clash of prejudices. The supporters of dark matter just don鈥檛 like the idea of abandoning conventional gravity, a creation of the two most idolised physicists, Newton and Einstein. MOND advocates don鈥檛 like the idea of postulating vast amounts of an unknown substance 鈥� new phlogiston. It all makes for happy times at the speculation factory. Science needs its bold hypotheses, but in this case the ratio of fact to fancy seems unusually small.
Is there an end in sight?
In the next few years, the best hope is probably that an astronomical observation will blow one or other side out of the water. Perhaps the James Webb telescope, Hubble鈥檚 successor, will probe deeply enough into galaxies to show that their gravitational fields have a completely different shape from the distribution of visible stars and gas. That would be a setback for MOND, which holds that ordinary matter is the only source of gravity. Even so, someone might then come up with an ingenious new kind of lopsided long-range force that will fit the data, but the case for dark matter would be immensely strengthened.
Yet even this wouldn鈥檛 tell us what dark matter is. That might come from an unequivocal detection of WIMPs or other exotic particles, perhaps in a sodium iodide crystal sitting at the bottom of a mineshaft in Minnesota or Yorkshire. It would be a fabulous discovery, but don鈥檛 hold your breath. The track record of similar searches is not good: we have been waiting a long time to see magnetic monopoles and gravitational waves, and in this case the problem is even trickier because we do not know what we are looking for. The experiments are set up to look for either WIMPs or other candidate particles called axions. What we really need is an experiment with a longer reach that could search for particles with a wide range of masses and other properties. It might be difficult and costly, but if dark matter keeps getting into trouble, the effort could be worth it.
For now, though, it might be worthwhile to keep a small bet on the existence of the dark stuff. Phlogiston and the ether are not the only examples from the past. Remember that other mysterious, invisible entity, Faraday鈥檚 magnetic field?