Self-Interacting Dark Matter from Gravitational Scattering

I show that gravitational scattering of dark-matter objects of 10^4 solar masses and speeds of 10 km/s, provides the cross-section per unit mass required in self-interacting dark matter models that alleviate the small-scale structure challenges to the collisionless cold dark matter model. For primordial objects of mass 10^4*(M_4) solar masses, moving at the velocity dispersion characteristic of dwarf galaxies, 10*(v_1) km/s, the cross-section per unit mass for gravitational scattering is 10*[M_4/(v_1)^4] cm^2/g. The steep decline in interaction with increasing velocity explains why self-interaction is not evident in data on massive galaxies and clusters of galaxies.