Dark matter’s existence is all but certain—astronomers believe it makes up about a quarter of the universe’s total energy content—yet its true identity has eluded us for decades. Two of the leading candidates for dark matter are the hypothetical particles ultralight axions and dark photons, which in the range studied here would be some 19 to 21 orders of magnitude lighter than the electron.

Conventional axion searches tend to involve converting them into photons with the help of strong laboratory magnets. However, research in a laboratory inherently limits the space over which such a field can be applied.

A collaborative team of researchers from Kyoto University, Hiroshima University and Nihon University realized that, by contrast, Earth’s own magnetic field spans a scale no laboratory could match. Their paper is published in the journal Progress of Theoretical and Experimental Physics.