  In our recently proposed quantum theory of gravity, the universe is made of
`atoms' of space-time-matter (STM). Planck scale foam is composed of STM atoms
with Planck length as their associated Compton wave-length. The quantum
dispersion and accompanying spontaneous localisation of these STM atoms amounts
to a cancellation of the enormous curvature on the Planck length scale.
However, an effective dark energy term arises in Einstein equations, of the
order required by current observations on cosmological scales. This happens if
we propose an extremely light particle having a mass of about $10^{-33} \ {\rm
eV/c^2}$, forty-two orders of magnitude lighter than the proton. The
holographic principle suggests there are about $10^{122}$ such particles in the
observed universe. Their net effect on space-time geometry is equivalent to
dark energy, this being a low energy quantum gravitational phenomenon. In this
sense, the observed dark energy constitutes evidence for quantum gravity. We
then invoke Dirac's large number hypothesis to also propose a dark matter
candidate having a mass halfway (on the logarithmic scale) between the proton
and the dark energy particle, i.e. about $10^{-12}\ {\rm eV/c^2}$.
