  Many models of composite dark matter feature a first-order confinement
transition in the early Universe, which would produce a stochastic background
of gravitational waves that will be searched for by future gravitational-wave
observatories. We present work in progress using lattice field theory to
predict the properties of such first-order transitions. Targeting SU(N)
Yang--Mills theories, this work employs the Logarithmic Linear Relaxation (LLR)
density of states algorithm to avoid super-critical slowing down at the
transition.
