  The tidal deformability and the radius of neutron stars are observables,
which have been used to constrain the neutron star equation of state and
explore the composition in neutron stars. We investigated the radius and tidal
deformability of dark matter admixed neutron stars (DANSs) by utilizing the
two-fluid TOV equations. Assuming that the dark matter modeled as ideal fermi
gas or self-interacting bosons, for a series of DANSs at a fixed mass, it is
shown that there exists the DANSs with smaller normal matter radii but larger
tidal deformabilities. This negative correlation does not exist in the normal
neutron stars.In other words, if the observation finds that the neutron stars
with a fixed mass exists such a situation, that is, having a smaller observed
radius but a larger tidal deformability, it will indicate the existence of dark
matter in neutron stars.In addition, the relevant neutron star observations can
also be used to constrain the dark matter parameters.
