  Interactions between dark matter and ordinary matter will transfer momentum,
and therefore give rise to a force on ordinary matter due to the dark matter
`wind.' We present a realistic model of dark matter where this force can be
maximal, meaning that an order-1 fraction of the dark matter momentum incident
on a target of ordinary matter is reflected. The model consists of light m_\phi
\sim eV scalar dark matter with an effective interaction \phi^2 \bar{\psi}\psi,
where \psi is an electron or nucleon field. If the coupling is repulsive and
sufficiently strong, the field \phi is excluded from ordinary matter, analogous
to the Meissner effect for photons in a superconductor. We show that there is a
large region of parameter space that is compatible with existing constraints,
and which can be probed by sensitive force experiments, such as satellite tests
of the equivalence principle and torsion balance experiments.
