  The oscillation of the neutron $n$ into mirror neutron $n'$, its partner from
dark mirror sector, can gradually transform an ordinary neutron star into a
mixed star consisting in part of mirror dark matter. The implications of the
reverse process taking place in the mirror neutron stars depend on the sign of
baryon asymmetry in mirror sector. Namely, if it is negative, as predicted by
certain baryogenesis scenarios, then $\bar{n}'-\bar{n}$ transitions create a
core of our antimatter gravitationally trapped in the mirror star interior. The
annihilation of accreted gas on such antimatter cores could explain the origin
$\gamma$-source candidates, with unusual spectrum compatible to
baryon-antibaryon annihilation, recently identified in the Fermi LAT catalog,
In addition, some part of this antimatter escaping after the mergers of mirror
neutron stars can produce the flux of cosmic antihelium and also heavier
antinuclei which are hunted in the AMS-02 experiment.
