  Empirical theories of Dark Matter like MOND gravity and of Dark Energy like
f(R) gravity were motivated by astronomical data. But could these theories be
branches rooted from a more general hence natural framework? Here we propose
the natural Lagrangian of such a framework based on simple dimensional analysis
and co-variant symmetry requirements, and explore various outcomes in a
top-down fashion. Our framework preserves the co-variant formulation of GR, but
allows the expanding physical metric be bent by a single new species of Dark
Fluid flowing in space-time. Its non-uniform stress tensor and current vector
are simply functions of a vector field of variable norm, resembling the
4-vector electromagnetic potential description for the photon fluid, but is
dark (e.g., by very early decoupling from the baryon-radiation fluid). The Dark
Fluid framework naturally branches into a continuous spectrum of theories with
Dark Energy and Dark Matter effects, including the $f(R)$ gravity, TeVeS-like
theories, Einstein-Aether and $\nu\Lambda$ theories as limiting cases. When the
vector field degenerates into a pure Higgs-like scalar field, we obtain the
physics for inflaton and quintessence. In this broad setting we emphasize the
non-constant dynamical field behind the cosmological constant effect, and
highlight plausible corrections beyond the classical MOND predictions. Choices
of parameters can be made to pass BBN, PPN, and causality constraints. The Dark
Fluid is inspired to unify/simplify the astronomically successful ingredients
of previous constructions: the desired effects of inflaton plus quintessence
plus Cold DM particle fields or MOND-like scalar field(s) are shown largely
achievable by one vector field only.
