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 $2.4\sigma$. The probability of so large a subcluster velocity in cosmological simulations estimated from the Hayashi-White fit is $\lesssim {10}^{-7}$. A long-range force with strength $\approx 0.4–1.2$ times that of gravity would provide the corresponding additional acceleration.