  A long range Weak Equivalence Principle (WEP) violating force between Dark
Matter (DM) particles, mediated by an ultralight scalar, is tightly constrained
by galactic dynamics and large scale structure formation. We examine the
implications of such a "dark force" for several terrestrial experiments,
including Eotvos tests of the WEP, direct-detection DM searches, and collider
studies. The presence of a dark force implies a non-vanishing effect in Eotvos
tests that could be probed by current and future experiments depending on the
DM model. For scalar singlet DM scenarios, a dark force of astrophysically
relevant magnitude is ruled out in large regions of parameter space by the DM
relic density and WEP constraints. WEP tests also imply constraints on the
Higgs-exchange contributions to the spin-independent (SI) DM-nucleus direct
detection cross-section. For WIMP scenarios, these considerations constrain
Higgs-exchange contributions to the SI cross-section to be subleading compared
to gauge-boson mediated contributions. In multicomponent DM scenarios, a dark
force would preclude large shifts in the rate for Higgs decay to two photons
associated with DM-multiplet loops that might otherwise lead to measurable
deviations at the LHC or a future linear collider. The combination of
observations from galactic dynamics, large scale structure formation, Eotvos
experiments, DM-direct-detection experiments, and colliders can further
constrain the size of new long range forces in the dark sector.
