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.
