If DM is realized at around the TeV-mass region or above, even the heaviest electroweak force carriers could act as long-range forces, leading to the existence of meta-stable DM bound states. The formation and subsequent decay of the latter further deplete the relic density during the freeze-out process on top of the Sommerfeld enhancement, allowing for larger DM masses. In this talk, I discuss how a dense and hot primordial plasma environment influences these quantum mechanical effects, leading to a more complete description and more precise prediction of the thermal WIMP relic abundance.