  We show that dark energy and dark matter can be described simultaneously by
ordinary Einstein gravity interacting with a single scalar field provided the
scalar field Lagrangian couples in a symmetric fashion to two different
spacetime volume-forms (covariant integration measure densities) on the
spacetime manifold - one standard Riemannian given by the square-root of the
determinant of the pertinent Riemannian metric and another non-Riemannian
volume-form independent of the Riemannian metric, defined in terms of an
auxiliary antisymmetric tensor gauge field of maximal rank. Integration of the
equations of motion of the latter auxiliary gauge field produce an a priori
arbitrary integration constant that plays the role of a dynamically generated
cosmological constant or dark energy. Moreover, the above modified scalar field
action turns out to possess a hidden Noether symmetry whose associated
conserved current describes a pressureless "dust" fluid which we can identify
with the dark matter completely decoupled from the dark energy. The form of
both the dark energy and dark matter that results from above class of models is
insensitive to the specific form of the scalar field Lagrangian. By adding
appropriate perturbation, which breaks the above hidden symmetry and along with
this it couples dark matter and dark energy, we also suggest a way to obtain
growing dark energy in the present universe's epoch without evolution
pathologies.
