  We explore in a systematic way the possibility that long-range quantum
gravity effects could play a role at galactic scales and could be responsible
for the phenomenology commonly attributed to dark matter. We argue that the
presence of baryonic matter breaks the scale symmetry of the de Sitter (dS)
spacetime generating an IR scale $r_0$, corresponding to the scale at which the
typical dark matter effects we observe in galaxies arise. It also generates a
huge number of bosonic excitations with wavelength larger than the size of the
cosmological horizon and in thermal equilibrium with dS spacetime. We show that
for $r\gtrsim r_0$ these excitations produce a new component for the radial
acceleration of stars in galaxies which leads to the result found by McGaugh
{\sl et al.} by fitting a large amount of observational data and with the MOND
theory. We also propose a generalized thermal equivalence principle and use it
to give another independent derivation of our result. Finally, we show that our
result can be also derived as the weak field limit of Einstein's general
relativity sourced by an anisotropic fluid.
