  We argue that discreteness at the Planck scale (naturally expected to arise
from quantum gravity) might manifest in the form of minute violations of
energy-momentum conservation of the matter degrees of freedom when described in
terms of (idealized) smooth fields on a smooth spacetime. In the context of
applications to cosmology such `energy diffusion' from the low energy matter
degrees of freedom to the discrete structures underlying spacetime leads to the
emergence of an effective dark energy term in Einstein's equations. We estimate
this effect using a (relational) hypothesis about the materialization of
discreteness in quantum gravity which is motivated by the strict observational
constraints supporting the validity of Lorentz invariance at low energies. The
predictions coming from simple dimensional analysis yield a cosmological
constant of the order of magnitude of the observed value without fine tuning.
