  Recent observational results for the masses and radii of some neutron stars
are in contrast with typical observations and theoretical predictions for
"normal" neutron stars. We propose that their unusual properties can be
interpreted as the signature of a dark matter core inside them. This
interpretation requires that the dark matter is made of some form of stable,
long-living or in general non-annihilating particles, that can accumulate in
the star. In the proposed scenario all mass-radius measurements can be
explained with one nuclear matter equation of state and a dark core of varying
relative size. This hypothesis will be challenged by forthcoming observations
and could eventually be a useful tool for the determination of dark matter.
