A light ray flying past a massive body bends slightly. For a neutron star, this bending is usually assumed to be the same as for a point with the same mass. But if the star is surrounded by an invisible cloud of dark matter, light passing through that cloud is deflected more weakly. The reason is simple: the cloud is spread out in space, and the light only feels the part of it that lies within a cylinder around the ray. The farther from the center, the less mass there is, and the weaker the bend.

The scientists built a model of a star where ordinary matter and dark matter coexist as two fluids that interact only through gravity. For light dark matter particles, pressure puffs them into a cloud beyond the visible surface. They then computed the light deflection angle using a geometric theorem. It turned out that the shape of the deflection weakening depends on the dark matter fraction and the cloud size. This makes it possible to distinguish dark matter from uncertainties in the properties of ordinary matter.