Snowmass2021 Cosmic Frontier: Synergies between dark matter searches and
  multiwavelength/multimessenger astrophysics
  This whitepaper focuses on the astrophysical systematics which are
encountered in dark matter searches. Oftentimes in indirect and also in direct
dark matter searches, astrophysical systematics are a major limiting factor to
sensitivity to dark matter. Just as there are many forms of dark matter
searches, there are many forms of backgrounds. We attempt to cover the major
systematics arising in dark matter searches using photons -- radio and gamma
rays -- to cosmic rays, neutrinos and gravitational waves. Examples include
astrophysical sources of cosmic messengers and their interactions which can
mimic dark matter signatures. In turn, these depend on commensurate studies in
understanding the cosmic environment -- gas distributions, magnetic field
configurations -- as well as relevant nuclear astrophysics. We also cover the
astrophysics governing celestial bodies and galaxies used to probe dark matter,
from black holes to dwarf galaxies. Finally, we cover astrophysical backgrounds
related to probing the dark matter distribution and kinematics, which impact a
wide range of dark matter studies. In the future, the rise of multi-messenger
astronomy, and novel analysis methods to exploit it for dark matter, will offer
various strategic ways to continue to enhance our understanding of
astrophysical backgrounds to deliver improved sensitivity to dark matter.
