More is Different: Non-Minimal Dark Sectors and their Implications for Particle Physics, Astrophysics, and Cosmology -- 13 Take-Away Lessons for Snowmass 2021

The phrase "more is different" is often used to refer to the new, unexpected collective phenomena that can arise when the number of states in a given system is large. In this contribution to the Snowmass 2021 Study, we describe 13 unexpected collective phenomena that can arise when the dark sector contains a large number of states, contrary to the usual assumptions. These 13 take-away lessons stretch across all of the domains of relevance for dark-matter physics, including collider signatures, direct-detection signatures, indirect-detection signatures, new perspectives on dark-matter complementarity, and even unexpected astrophysical and cosmological phenomena that transcend those normally associated with single-component dark-matter scenarios. These lessons -- and the phenomena on which they are based -- thereby illustrate the need to maintain a broad perspective when contemplating the possible signatures and theoretical possibilities associated with non-minimal dark sectors.