  We study supersymmetric scenarios where the dark matter is the gaugino of an
unbroken hidden U(1) which interacts with the visible world only via a small
kinetic mixing with the hypercharge. Strong constraints on the parameter space
can be derived from avoiding overclosure of the Universe and from requiring
successful Big Bang Nucleosynthesis and structure formation. We find that for
typical values of the mixing parameter, scenarios with neutralino NLSP are
excluded, while scenarios with slepton NLSP are allowed when the mixing
parameter lies in the range chi~O(10^(-13) - 10^(-10)). We also show that if
the gravitino is the LSP and the hidden U(1) gaugino the NLSP, the bounds on
the reheating temperature from long lived charged MSSM relics can be
considerably relaxed and we comment on the signatures of these scenarios at
future colliders. Finally, we discuss the case of an anomalously small mixing,
chi<<10^(-16), where the neutralino becomes a decaying dark matter candidate,
and derive constraints from gamma ray experiments.
