  We revisit the possibility that Dark Matter is composed of stable scalar
glueballs of a confining dark ${\rm SU}(3)$ gauge theory coupled only to
gravity. The relic abundance of dark glueballs is studied for the first time in
a thermal effective theory accounting for strong-coupling dynamics. An
important ingredient of our analysis is the use of an effective potential for
glueballs that is fitted by lattice simulations. We predict the relic abundance
to be in the range $0.12\zeta_{T}^{-3}\Lambda/(137.9 {\rm eV}) \lesssim \Omega
h^{2}\lesssim 0.12\zeta_{T}^{-3}\Lambda/(82.7 {\rm eV})$, with $\Lambda$ being
the confinement scale, $\zeta_{T}$ the visible-to-dark sector temperature ratio
and the uncertainty is coming from the fit to lattice data. This prediction is
an order of magnitude smaller than the existing glueball abundance results in
the literature. Our framework can be easily generalised to different gauge
groups and modified cosmological histories paving the way towards consistent
exploration of strongly-coupled dark sectors and their cosmological
implications.
