  We show that our Universe may be inhomogeneous on large sub-horizon scales
without us being able to realise it. We assume that a network of domain walls
permeates the universe dividing it in domains with slightly different vacuum
energy densities. We require that the energy scale of the phase transition
which produced the domain walls is sufficiently low so that the walls have a
negligible effect on structure formation. Nevertheless, the different vacuum
densities of different domains will lead to different values of the
cosmological parameters $\Omega_\Lambda^0$, $\Omega_m^0$ and $h$, in each patch
thus affecting the growth of cosmological perturbations at recent times. Hence,
if our local patch of the universe (with uniform vacuum density) is big enough
-- which is likely to happen given that we should have on average about one
domain per horizon volume -- we might not notice these large-scale
inhomogeneities. This happens because in order to see a patch with a different
vacuum density one may have to look back at a time when the universe was still
very homogeneous.
