  Multi-level dark matter with diagonal and off-diagonal interactions shows a
rich phenomenology in its self-scattering. If the interactions are mediated by
a particle that is less massive than the dark matter, Sommerfeld effect can
lead to resonant enhancement of the scattering. For mediators lighter than the
level separation, dark matter particles can upscatter to excited states and
de-excite by emitting these mediators. We compute these cross-sections, both
above and below the kinematic threshold, in a generic two-component dark matter
model and identify the large inelastic cross-section as a result of maximal
mixing between the two states. A new route for cooling of large dark matter
halos and a new drag force between two colliding halos are identified and shown
to arise purely from the inelastic scattering.
