  In this work, we show that a large class of models with a composite dark
sector undergo a strong first order phase transition in the early universe,
which could lead to a detectable gravitational wave signal. We summarise the
basic conditions for a strong first order phase transition for SU(N) dark
sectors with n_f flavours, calculate the gravitational wave spectrum and show
that, depending on the dark confinement scale, it can be detected at eLISA or
in pulsar timing array experiments. The gravitational wave signal provides a
unique test of the gravitational interactions of a dark sector, and we discuss
the complementarity with conventional searches for new dark sectors. The
discussion includes Twin Higgs and SIMP models as well as symmetric and
asymmetric composite dark matter scenarios.
