  We study systematically which features in the cosmic microwave background
(CMB) and large-scale structure (LSS) probe various inhomogeneous properties of
the dark sectors (including neutrinos, dark matter, and dark energy). We
stress, and quantify by simple formulas, that the primary CMB anisotropies are
very susceptible to the gravitational potentials during horizon entry, less at
recombination. The CMB thus allows us to scan \Phi+\Psi and the underlying dark
kinetics for all redshifts z~1-10^5. LSS, on the other hand, responds strongest
to \Phi at low redshifts. Dark perturbations are often parameterized by the
anisotropic stress and effective sound speed (stiffness). We find that the dark
anisotropic stress and stiffness influence the visible species at the
correspondingly early and late stages of horizon entry, and affect stronger
respectively the CMB and LSS. The CMB yet remains essential to probing the
stiff perturbations of light neutrinos and dark energy, detectable only during
horizon entry. The clustering of dark species and large propagation speed of
their inhomogeneities also map to distinctive features in the CMB and LSS. -Any
parameterization of the signatures of dark kinetics that assumes general
relativity can effectively accommodate any modified gravity (MG) that retains
the equivalence principle for the visible sectors. This implies that formally
the nonstandard structure growth or \Phi/\Psi ratio, while indicative, are not
definitive MG signatures. The definitive signatures of MG may include the
strong dependence of the apparent dark dynamics on visible species, its
superluminality, and the nonstandard phenomenology of gravitational waves.
