Alternative unified views on dark matter and dark energy

The simple fact that the present closure contributions in dark matter and dark energy are nearly equal begs the question as to whether they could be different aspects of the same physical phenomenon. Here, we review constraints several postulates as to how this coincidence could be achieved. These include: 1) The possibility that the dark matter decays producing a bulk viscosity in the cosmic fluid; 2) cosmic acceleration produced by the inflow of dark matter from a bulk dimension in brane-world cosmology; and 3) The possibility of relativistic corrections to the Friedmann equation from the presence of local inhomogeneities. Constraints and observational tests of each of these cosmologies are proposed.