  After reviewing the modified Newtonian dynamics (MOND) proposal, we advocate
that the associated phenomenology may actually not result from a modification
of Newtonian gravity, but from a mechanism of "gravitational polarization" of
some dipolar medium playing the role of dark matter. We then build a
relativistic model within standard general relativity to describe (at some
phenomenological level) the dipolar dark matter polarizable in a gravitational
field. The model naturally involves a cosmological constant, and is shown to
reduce to the concordance cosmological scenario (Lambda-CDM) at early
cosmological times. From the mechanism of gravitational polarization, we
recover the phenomenology of MOND in a typical galaxy at low redshift.
Furthermore, we show that the cosmological constant Lambda scales like a0^2,
where a0 is the constant MOND acceleration scale, in good agreement with
observations.
