  We examine the possibility of electroweak baryogenesis and dark matter in the
nMSSM, a minimal extension of the MSSM with a singlet field. This extension
avoids the usual domain wall problem of the NMSSM, and also appears as the low
energy theory in models of dynamical electroweak symmetry breaking with a
so-called fat-Higgs boson. We demonstrate that a strong, first order
electroweak phase transition, necessary for electroweak baryogenesis, may arise
in regions of parameter space where the lightest neutralino provides an
acceptable dark matter candidate. We investigate the parameter space in which
these two properties are fulfilled and discuss the resulting phenomenology. In
particular, we show that there are always two light CP-even and one light
CP-odd Higgs bosons with masses smaller than about 250 GeV. Moreover, in order
to obtain a realistic relic density, the lightest neutralino mass tends to be
smaller than $M_Z/2$, in which case the lightest Higgs boson decays
predominantly into neutralinos.
