  A spontaneously broken hidden U(1)$_h$ gauge symmetry can explain both the
dark matter stability and the observed relic abundance. In this framework, the
light gauge boson can mediate the strong dark matter self-interaction, which
addresses astrophysical observations that are hard to explain in collisionless
cold dark matter. Motivated by flavoured grand unified theories, we introduce
right-handed neutrinos and a flavoured $B - L$ gauge symmetry for the third
family U(1)$_{(B-L)_3}$. The unwanted relic of the U(1)$_h$ gauge boson decays
into neutrinos via the kinetic mixing with the U(1)$_{(B - L)_3}$ gauge boson.
Indirect detection bounds on dark matter are systematically weakened, since
dark matter annihilation results in neutrinos. However, the kinetic mixing
between U(1)$_{(B - L)_3}$ and U(1)$_Y$ gauge bosons are induced by quantum
corrections and leads to an observable signal in direct and indirect detection
experiments of dark matter. This model can also explain the baryon asymmetry of
the Universe via the thermal leptogenesis. In addition, we discuss the
possibility of explaining the lepton flavour universality violation in
semi-leptonic $B$ meson decays that is recently found in the LHCb experiment.
