  We explore the phenomenology of a two-fluid cosmological model, where the
field equations of general relativity (GR) are sourced by baryonic and cold
dark matter. We find that the model allows for a unified description of small
and large scale, late-time cosmological dynamics. Specifically, in the static
regime we recover the flattening of galactic rotation curves by requiring the
matter density profile to scale as $1/r^2$. The same behavior describes matter
inhomogeneities distribution at small cosmological scales. This traces galactic
dynamics back to structure formation. At large cosmological scales, we focus on
back reaction effects of the spacetime geometry to the presence of matter
inhomogeneities. We find that a cosmological constant with the observed order
of magnitude, emerges by averaging the back reaction term on spatial scales of
order 100 Mpc and it is related in a natural way to matter distribution. This
provides a resolution to both the cosmological constant and the coincidence
problems and shows the existence of an intriguing link between the small and
large scale behavior in cosmology.
