Is Self-Interacting Dark Matter Undergoing Dark Fusion?
  We suggest that two-to-two dark matter fusion may be the relaxation process
that resolves the small-scale structure problems of the cold collisionless dark
matter paradigm. In order for the fusion cross section to scale correctly
across many decades of astrophysical masses from dwarf galaxies to galaxy
clusters, we require the fractional binding energy released to be greater than
v^n ~ [10^{-(2-3)}]^n, where n=1,2 depends on local dark sector chemistry. The
size of the dark-sector interaction cross sections must be sigma ~ 0.1-1 barn,
moderately larger than for Standard Model deuteron fusion, indicating a dark
nuclear scale Lambda ~ O(100 MeV). Dark fusion firmly predicts constant sigma v
below the characteristic velocities of galaxy clusters. Observations of the
inner structure of galaxy groups with velocity dispersion of several hundred
kilometer per second, of which a handful have been identified, could
differentiate dark fusion from a dark photon model.
