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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 (Tolman-Oppenheimer-Volkoff) equation. Assuming that the dark matter modeled as fermions (with or without self-interaction) 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.
Sun et al. (Mon,) studied this question.