We discuss cosmological constraints on glueballs arising from hidden Yang-Mills sectors. These are well-motivated from UV physics such as string theory. As it turns out, dark glueballs are overproduced for large regions of ultraviolet parameter space. After explaining this dark glueball problem we address several ways in which the problem can be alleviated, e.g. via preferential reheating of the visible sector or glueball decay into lighter particles. Furthermore, we discuss the constraints placed on these alleviation mechanisms from experimental bounds.