  Dark matter constitutes $26\%$ of the total energy in our universe, but its
nature remains elusive. Among the assortment of viable dark matter candidates,
particles and fields with masses lighter than $40 \mathrm{eV}$, called
ultralight dark matter, stand out as particularly promising thanks to their
feasible production mechanisms, consistency with current observations, and
diverse and testable predictions. In light of ongoing and forthcoming
experimental and observational efforts, it is important to advance the
understanding of ultralight dark matter from theoretical and phenomenological
perspectives: How does it interact with itself, ordinary matter, and gravity?
What are some promising ways to detect it?
  In this thesis, we aim to explore the dynamics and interaction of ultralight
dark matter and other astrophysically accessible hypothetical fields in a
relatively model-independent way. Without making specific assumptions about
their ultraviolet physics, we first demonstrate a systematic approach for
constructing a classical effective field theory for both scalar and vector dark
fields and discuss conditions for its validity. Then, we explore the
interaction of ultralight dark fields, both gravitational and otherwise, within
various contexts such as nontopological solitons, neutron stars, and
gravitational waves.
