A neutron star is a city the size of Manhattan that weighs as much as half a million Earths. A teaspoon of its material would outweigh Mount Everest. Now, into this superdense sphere, something invisible and elusive has been mixed — dark matter particles.

That's exactly what physicists did in a new study. They took three different recipes (equations of state) for ordinary neutron star matter and added a bit of dark matter to each — light neutralinos that interact with ordinary matter via Higgs boson exchange. The amount of dark matter was set by a single parameter: its Fermi momentum (the maximum energy of the particles). The larger it is, the more dark matter in the star.

Then the scientists solved the equations linking pressure and density and obtained how the mass, radius, and tidal deformability of the star change. Tidal deformability shows how easily a star changes shape under the gravity of a neighbor, like a ball that squeezes in your hand.

The result was clear: dark matter softens the star. As its fraction increases, the maximum mass drops, the radius at 1.4 solar masses decreases, and the tidal deformability sharply declines. For example, for one of the recipes, at the maximum dark matter fraction, the deformability dropped 8 times more than the typical scatter from uncertainties in the composition of ordinary matter.

This means dark matter can be searched for simply by observing neutron stars. If the measured deformability turns out to be significantly lower than models without dark matter predict, that would be its signature. Moreover, the effect is so strong that it cannot be confused with ordinary uncertainties in our knowledge of stellar matter. Thus, neutron stars become a laboratory for searching the invisible.