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Still searching (Image: Joseba Elorza)

WE HAVE been aware of the need for dark matter since the 1930s. Without this stuff, we can鈥檛 make sense of the rotation of galactic clusters, or how galaxies formed in the first place. And yet, to date, we have found nothing. Even CERN鈥檚 Large Hadron Collider, our best and by far most expensive tool for finding it, has so far drawn a blank. How much longer can we keep looking?

Dark matter is posited as the resolution to several obstinate anomalies 鈥� as Isaac Asimov put it, things that make you exclaim not 鈥淓ureka!鈥� but 鈥渢hat鈥檚 funny鈥︹�� Such anomalies are often the key to scientific progress. But dark matter, and our efforts to pin it down, have been around long enough for doubts to creep in.

Perhaps we have simply been looking for the wrong thing. Perhaps dark matter particles are very massive, rather than fairly light, as many assume. The first experiments are now under way to detect any such 鈥渟uperheavy鈥� dark matter that might have been created when the universe was just getting started (see 鈥�WIMPZILLAs: Monster particles from the dawn of time鈥�).

聯Perhaps dark matter鈥檚 true identity is so unexpected that we haven鈥檛 even thought to look for it聰

Or perhaps the true identity of dark matter is so unexpected that we haven鈥檛 even thought to look for it, despite potential evidence lurking somewhere in the vast quantities of data from the LHC.

Or perhaps we have embarked on one of those quixotic quests that mark the history of physics. At the beginnings of cosmology, Ptolemy devised a model of planetary motion that closely fitted observations. For more than a millennium, his successors adjusted these 鈥渆picycles鈥� for new-found anomalies. Their laudable commitment and ingenuity was to increasingly little effect. In the end, Copernicus and Kepler blew the whole thing away 鈥� though it took a while for their model to be accepted.

A more recent parallel comes from the search for luminiferous ether, the all-pervading substance once thought to be the medium for light. When Albert Michelson and Edward Morley failed to detect it in 1887, they didn鈥檛 declare that the world needed a new theory for the propagation of light. Instead, they and others built a series of bigger and better instruments to find it. Eventually special relativity abolished the anomaly 鈥� but many etherists carried on looking regardless.

Can we be sure we鈥檙e not in their position, looking for something that isn鈥檛 there? Well, there is no robust alternative to dark matter; plans to resolve the cosmic anomalies by other means, such as modified gravity are not well-attested.

But pragmatically, the real issue is not the science, but the money. Most physicists would say it鈥檚 worth persevering with the search, given its potentially huge ramifications. But how long can they persuade their funders to keep paying for it? Consider the search for extraterrestrial intelligence, which, despite its potential impact, now receives no public money and relatively little private support. That鈥檚 easily explained: the scale of the task and the limits of our technology mean the chances of finding intelligent aliens seem slim. Given a few more years of null results, dark matter might come to seem a less worthwhile investment to cash-strapped funding bodies too.

We鈥檒l have to hope that doesn鈥檛 happen before we find either dark matter, some fresh anomalies that upset the apple cart, or a clever new resolution to our cosmic conundrums. And we鈥檒l also have to hope that, unlike the epicyclists and etherists, we鈥檒l recognise that breakthrough promptly when it comes.

This article appeared in print under the headline 鈥淣o end in sight鈥�