Cosmological observations reveal that our understanding of the fundamental particles and forces of nature is vastly incomplete. For example, we don’t yet know the particle nature of 84% of the matter in the universe, or of the processes that seeded the structure we observe in the universe today. In this talk I will introduce an under-explored class of dark matter scenarios with striking signatures in gravitational waves, give an update on the status of early-universe observations, and explain how an open quantum systems framework provides a rich new set of ideas for the theoretical foundations of cosmology.