Dark Silicon refers to the constraint that only a fraction of on-chip resources (cores) can be simultaneously powered-on (running at full performance) in order to stay within the allowable power budget and safe temperature limits, while others remain 'dark'. In this paper, we demonstrate how these 'dark cores' can be leveraged to improve the temperature profile at run-time, thus providing opportunities to power-on more cores at the nominal voltage than the number allowed when strictly obeying the conventional Thermal Design Power (TDP) constraint. In this paper, we propose a computationally efficient dark silicon management technique that determines the best set of cores to keep dark and the mapping of threads to cores at run-time, while also accounting for the impact of process variations. We have developed a lightweight temperature prediction mechanism that determines the impact of different candidate solutions on the chip thermal profile. Experimental evaluation of the proposed techniques on a simulated 8×8 many-core processor, and across a range of chips to account for process variations, show that the total instruction throughput is increased by 1.8× on average while keeping the temperature within the safe limits, when compared with state-of-the-art approaches.