  We demonstrate that dark matter particles gravitationally bound to the Earth
can induce a characteristic nuclear recoil signal at low energies in direct
detection experiments. The new spectral feature we predict can provide the
ultimate smoking gun for dark matter discovery for experiments with positive
signal but unclear background. The new feature is universal, in that the ratio
of bound over halo dark matter event rates at detectors is independent of the
dark matter-nucleon cross section.
