  Motivated by the null results of current dark matter searches and the
small-scale problems, we study a dark sector charged by a spontaneous broken
gauge $U(1)'$. To explore the parameter space of this model, in addition to the
consideration of the small-scale data, we also consider the theoretical bounds
on the dark Higgs mass, with the upper bound coming from the tree-level
perturbative unitarity and the lower bound from the one-loop Linde-Weinberg
bound. We deeply examine the dependence of the Linde-Weinberg bound on gauge
choice and energy scale, and present a Linde-Weinberg bound that is gauge and
scale independent. Combining the theoretical and observational constraints, we
obtain the following ranges for the parameter space: the dark matter mass is
10-500 GeV, the mediator (dark photon) mass is 0.5-5 MeV, the dark Higgs mass
is 0.05-50 MeV, and the dark fine-structure constant is 0.001-0.4. We conclude
that the dark Higgs in this model cannot be ignored in the phenomenological
study of the dark sector.
