Characterizing the structure and evolution of the cool gas in dark matter halos is a key element in current models of galaxy formation. Several physical mechanisms have been introduced to explain the origin of the cool, T ∼ 1e4 K gas in the dark matter halos of normal galaxies. Some of these mechanisms include cold accretion from the IGM, gas-rich galaxy mergers, starburst driven outflows, and thermal instabilities generated within the hot coronal gas. Unfortunately, direct empirical data are lacking and these mechanisms remain poorly constrained. My talk focuses on the empirical determination of the cool gas content of massive dark matter and directly addresses the physical origin of the gas. Specifically, I will present a series of empirical constraints on the cool gas content of dark matter halos hosting Luminous Red Galaxies (LRGs) at z ≈ 0.5. In my talk, I will describe the outcome of a clustering analysis done on a population of MgII absorption doublets and its implications for the presence of cool gas in massive halos. I will then describe the main results of an on-going spectroscopic survey of LRGs and present measurements of the covering fraction of cool gas in these galaxies. I will also present the results of a stellar population synthesis analysis done on a sample of spectroscopic LRGs. This analysis allows me to directly assess whether or not there is a connection between the presence of the gas and the recent star formation activities of the galaxies. Finally, I will briefly introduce on-going projects to fully explore the physical origin of the gas in more details.