  Motivated by the observed cosmic matter distribution, we present the
following conjecture: due to the formation of voids and opaque structures, the
average matter density on the path of the light from the well-observed objects
changes from Omega_M ~ 1 in the homogeneous early universe to Omega_M ~ 0 in
the clumpy late universe, so that the average expansion rate increases along
our line of sight from EdS expansion Ht ~ 2/3 at high redshifts to free
expansion Ht ~ 1 at low redshifts. To calculate the modified observable
distance-redshift relations, we introduce a generalized Dyer-Roeder method that
allows for two crucial physical properties of the universe: inhomogeneities in
the expansion rate and the growth of the nonlinear structures. By treating the
transition redshift to the void-dominated era as a free parameter, we find a
phenomenological fit to the observations from the CMB anisotropy, the position
of the baryon oscillation peak, the magnitude-redshift relations of type Ia
supernovae, the local Hubble flow and the nucleosynthesis, resulting in a
concordant model of the universe with 90% dark matter, 10% baryons, no dark
energy, 15 Gyr as the age of the universe and a natural value for the
transition redshift z_0=0.35. Unlike a large local void, the model respects the
cosmological principle, further offering an explanation for the late onset of
the perceived acceleration as a consequence of the forming nonlinear
structures. Additional tests, such as quantitative predictions for angular
deviations due to an anisotropic void distribution and a theoretical derivation
of the model, can vindicate or falsify the interpretation that light
propagation in voids is responsible for the perceived acceleration.
