  We study in this work a scenario that the universe undergoes a two step phase
transition with the first step happened to the dark matter sector and the
second step being the transition between the dark matter and the electroweak
vacuums, where the barrier between the two vacuums, that is necessary for a
strongly first order electroweak phase transition (EWPT) as required by the
electroweak baryogenesis mechanism, arises at the tree-level. We illustrate
this idea by working with the standard model (SM) augmented by a scalar singlet
dark matter and an extra scalar singlet which mixes with the SM Higgs boson. We
study the conditions for such pattern of phase transition to occur and
especially for the strongly first order EWPT to take place, as well as its
compatibility with the basic requirements of a successful dark matter, such as
observed relic density and constraints of direct detections. We further explore
the discovery possibility of this pattern EWPT by searching for the
gravitational waves generated during this process in spaced based
interferometer, by showing a representative benchmark point of the parameter
space that the generated gravitational waves fall within the sensitivity of
eLISA, DECIGO and BBO.
