  We run high resolution fully GPU N-body simulations to demonstrate that the
dark matter distribution in M31 is well-fitted by a core-like profile. Rich
observational data especially on the giant stellar stream provides stringent
constraints on the initial conditions of our simulations. We demonstrate that
accretion of a satellite on a highly eccentric orbit heats up the central parts
of M31, causes an outward migration of dark matter particles, flattens the
central cusp over more than a decade in scale and generates a new
model-independent dark matter profile that is well-fitted by a core. Our
results imply that core-like central profiles could be a common feature of
massive galaxies that have been initially cuspy but have accreted satellites on
nearly radial orbits.
