A New Force in the Dark Sector?

We study the kinematics of dark matter using the massive cluster of galaxies 1E0657-56. The velocity of the ``bullet'' subcluster (actually, the bow shock velocity) has been measured by x-ray emission from the shock front, and the masses and separation of the main and subclusters have been measured by gravitational lensing. The velocity with gravity alone is calculated in a variety of models of the initial conditions, mass distribution, and accretion history; it is much higher than the velocity of the bow shock, by at least 2.4σ. The probability of so large a subcluster velocity in cosmological simulations estimated from the Hayashi-White fit is 10^-7. A long-range force with strength ≈0.4--1.2 times that of gravity would provide the corresponding additional acceleration.