  We study cosmological constraints on dark pure Yang-Mills sectors. Dark
glueballs are overproduced for large regions of ultraviolet parameter space.
The problem may be alleviated in two ways: via a large preferential reheating
into the visible sector, motivating certain inflation or modulus decay models,
or via decays into axions or moduli, which are strongly constrained by
nucleosynthesis and $\Delta N_{\text{eff}}$ bounds. String models frequently
have multiple hidden Yang-Mills sectors, which are subject to even stronger
constraints due to the existence of multiple dark glueballs.
