  We investigate a class of models for dark matter and/or negative-pressure,
dynamical dark energy consisting of ``spintessence,'' a complex scalar field
$\phi$ spinning in a U(1)-symmetric potential $V(\phi)=V(|\phi|)$. As the
Universe expands, the field spirals slowly toward the origin. The internal
angular momentum plays an important role in the cosmic evolution and
fluctuation dynamics. We outline the constraints on a cosmic spintessence
field, describing the properties of the potential necessary to sustain a viable
dark energy model, making connections with quintessence and self-interacting
and fuzzy cold dark matter. Possible implications for the coincidence problem,
baryogenesis, and cosmological birefringence, and generalizations of
spintessence to models with higher global symmetry and models in which the
symmetry is not exact are also discussed.

Astrophysics;General Relativity and Quantum Cosmology;High Energy Physics - Phenomenology;High Energy Physics - Theory