  This paper proposes a strategy for detecting the presence of a
gravito-magnetic field due to the rotation of the galactic dark halo. Visible
matter in galaxies rotates and dark matter, supposed to form a halo
incorporating barionic matter, rotates also, since it interacts gravitationally
with the rest. Pursuing the same line of reasoning, dark matter should produce
all gravitational effects predicted by general relativity, including a
gravito-magnetic field. I discuss a possible strategy for measuring that field.
The idea recovers the old Sagnac effect and proposes to use a triangle having
three Lagrange points of the Sun-Earth pair at its vertices. The asymmetry in
the times of flight along the loop in opposite directions is proportional to
the gravito-magnetic galactic field.
