Possible nature of Dark Matter
  {We present a study of neutron star models that contain dark matter (DM) in
the core. The DM is assumed to have a particle nature and to be
self-interacting. Using constraints on the mass and radius of neutron stars, we
investigate the allowed properties of either bosonic or fermionic DM particles.
We consider cases where it constitutes up to 15\% of the mass of the star, even
though conventional mechanisms cannot generate such large fractions. For this
purpose three different models of neutron stars are considered, the first
involving nucleons only, the second including hyperons, and the last involving
strange matter in the core. Different EoSs are constructed for the various
cases of fermionic and bosonic DM. These EoSs are solved for selected
properties of the DM particles and the results are tested against mass, radius
and tidal deformability constraints for neutron stars. The distribution of
energy density of DM and ordinary matter inside the neutron stars is also
presented. It is found that if the DM is fermionic in nature it does not just
sit in the core but it is present everywhere in the star, from the centre to
outside the surface and may even envelop it.
