  A clump moving through the intracluster medium of a galaxy cluster can drive
a bow shock trailed by a bullet-like core. In some cases, such as in the
prototypical Bullet cluster, X-rays show a gas bullet with a protruding head
and pronounced shoulders. We point out that these features, while difficult to
explain without dark matter (DM), naturally arise as the head of the
slowed-down gas is gravitationally pulled forward toward its unhindered DM
counterpart. X-ray imaging thus provides a unique, robust probe of the offset,
collisionless DM, even without gravitational lensing or other auxiliary data.
Numerical simulations and a toy model suggest that the effect is common in
major mergers, is often associated with a small bullet-head radius of
curvature, and may lead to distinct bullet morphologies, consistent with
observations.
