  Even though the sensitivity of direct dark matter search experiments reach
the level about $10^{-45}~{\rm cm}^2$, there is no confident signal of dark
matter been observed. We point out that, if dark matter is a vector boson, the
null result in direct dark matter search experiments may due to the destructive
effects in dark-matter-nucleon elastic scattering. We illustrate the scenario
using a modified Higgs portal model that includes exotic quarks. The
significant cancellation can occur for certain mass gap between heavy quark and
dark matter. As a result, the spin-independent dark-matter-nucleon elastic
scattering is so suppressed that the future direct search experiments can
hardly observe the signal of dark matter.
