How a simple physics experiment could reveal dark matter hiding in an extra dimension

We tend not to dwell on the fact that we exist in three dimensions. Forwards-back, left-right, up-down; these are the axes on which we navigate the world. When we try to imagine something else, it typically conjures images from the wildest science fiction – of portals in the fabric of space-time and parallel worlds.

Yet serious physicists have long been spellbound by the prospect of extra dimensions. For all their intangibility, they promise to resolve several big questions about the deepest workings of the universe. Besides, they can’t be ruled out simply because they are difficult to imagine and even harder to observe. “There’s no reason why it has to be three,” says Georges Obied at the University of Oxford. “It could have been two; it could have been four or 10.”

Still, there comes a point when any self-respecting physicist wants hard evidence. Which is why it is so exciting that, over the past few years, researchers have developed a handful of techniques that could finally snare proof of extra dimensions. We might yet spot gravity leaking into them, for instance. We may see their subtle imprint on black holes or find their traces in particle accelerators.

But now, in an unexpected twist, Obied and others are making the case for an extra dimension that is radically unlike any we have concocted previously. This “dark dimension” would conceal particles from the dawn of time that could solve the mystery of dark matter, whose gravitational pull is thought to have shaped the cosmos. Crucially, it should also be relatively…