  SUSY/SUGRA models with naturalness defined via small $\mu$ are constrained
due to experiment on the relic density and the experimental limits on the
WIMP-proton cross-section and WIMP annihilation cross-section from indirect
detection experiments. Specifically models with small $\mu$ where the
neutralino is higgsino-like lead to dark matter relic density below the
observed value. In several works this problem is overcome by assuming dark
matter to be constituted of more than one component and the neutralino relic
density deficit is made up from contributions from other components. In this
work we propose that the dark matter consists of just one component, i.e., the
lightest neutralino and the relic density of the higgsino-like neutralino
receives contributions from the usual freeze-out mechanism along with
contributions arising from the decay of hidden sector neutralinos. The model we
propose is an extended MSSM model where the hidden sector is constituted of a
$U(1)_X$ gauge sector along with matter charged under $U(1)_X$ which produce
two neutralinos in the hidden sector. The $U(1)_X$ and the hypercharge $U(1)_Y$
of the MSSM have kinetic and Stueckelberg mass mixing where the mixings are
ultraweak. In this case the hidden sector neutralinos have ultraweak
interactions with the visible sector. Because of their ultraweak interactions
the hidden sector neutralinos are not thermally produced and we assume their
initial relic density to be negligible. However, they can be produced via
interactions of MSSM particles in the early universe, and once produced they
decay to the neutralino. For a range of mixings the decays occur before the BBN
producing additional relic density for the neutralino. Models of this type are
testable in dark matter direct and indirect detection experiments and at the
high luminosity and high energy LHC.
