The results are inconclusive, but, if combined with recent theory, they hint at something exciting. Could the universe have a dark side, complete with its own force, a zoo of particles and even a shadow version of the Milky Way?

"There could be a mirror world where interesting things are going on," says James Bullock of the University of California at Irvine, who has been working on the idea of a "dark sector" for a while. "It means nature is much richer than we would otherwise know," he says.

The dark sector could help explain why we've failed to detect dark matter on Earth so far, but it would also demand a radical shift in our understanding of the stuff.