We examine the neutralino relic density in the presence of a light top squark, such as the one required for the realization of the electroweak baryogenesis mechanism, within the minimal supersymmetric standard model. We show that there are three clearly distinguishable regions of parameter space, where the relic density is consistent with WMAP and other cosmological data. These regions are characterized by annihilation cross sections mediated by either light Higgs bosons, Z bosons, or by the co-annihilation with the lightest top squark. Tevatron collider experiments can test the presence of the light top squark in most of the parameter space. In the co-annihilation region, however, the mass difference between the light top squark and the lightest neutralino varies between 15 and 30 GeV, presenting an interesting challenge for top squark searches at hadron colliders. We present the prospects for direct detection of dark matter, which provides a complementary way of testing this scenario. We also derive the required structure of the high energy soft supersymmetry breaking mass parameters where the neutralino is a dark matter candidate and the top squark spectrum is consistent with electroweak baryogenesis and the present bounds on the lightest Higgs boson mass.