  We study a conformal version of the Standard Model (SM), which apart from SM
sector, containing a $ U_{D}(1) $ dark sector with a vector dark matter
candidate and a scalar field (scalon). In this model the dark sector couples to
the SM sector via a Higgs portal. The theory is scale-invariant in lowest
order, therefore the spontaneous symmetry breaking of scale invariance entails
the existence of a scalar particle, scalon, with vanishing zeroth-order mass.
However, one-loop corrections break scale invariance, so they give mass to the
scalon. Because of the scale invariance, our model is subjected to constraints
which remove many of the free parameters. We put constraints to the two
remaining parameters from the Higgs searches at the LHC, dark matter relic
density and dark matter direct detection limits by PandaX-II. The viable mass
region for dark matter is about 1-2 TeV. We also obtain the finite temperature
one-loop effective potential of the model and demonstrate that finite
temperature effects, for the parameter space constrained by dark matter relic
density, induce a strongly first-order electroweak phase transition.
