  Dark Yang-Mills sectors, which are ubiquitous in the string landscape, may be
reheated above their critical temperature and subsequently go through a
confining first-order phase transition that produces stochastic gravitational
waves in the early universe. Taking into account constraints from lattice and
from Yang-Mills (center and Weyl) symmetries, we use a phenomenological model
to construct an effective potential of the semi quark-gluon plasma phase, from
which we compute the gravitational wave signal produced during confinement for
numerous gauge groups. The signal is maximized when the dark sector dominates
the energy density of the universe at the time of the phase transition. In that
case, we find that it is within reach of the next-to-next generation of
experiments (BBO, DECIGO) for a range of dark confinement scales near the weak
scale.
