  The method of non-Riemannian (metric-independent) spacetime volume-forms
(alternative generally-covariant integration measure densities) is applied to
construct a modified model of gravity coupled to a single scalar field
providing an explicit unification of dark energy (as a dynamically generated
cosmological constant) and dust fluid dark matter flowing along geodesics as an
exact sum of two separate terms in the scalar field energy-momentum tensor. The
fundamental reason for the dark species unification is the presence of a
non-Riemannian volume-form in the scalar field action which both triggers the
dynamical generation of the cosmological constant as well as gives rise to a
hidden nonlinear Noether symmetry underlying the dust dark matter fluid nature.
Upon adding appropriate perturbation breaking the hidden "dust" Noether
symmetry we preserve the geodesic flow property of the dark matter while we
suggest a way to get growing dark energy in the present universe' epoch free of
evolution pathologies. Also, an intrinsic relation between the above modified
gravity + single scalar field model and a special quadratic purely kinetic
"k-essence" model is established as a weak-versus-strong-coupling duality.
