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 } \sim [ 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 \sim 0.1 - 1 \operatorname { barn } , moderately larger than for Standard Model deuteron fusion , indicating a dark nuclear scale \Lambda \sim { \cal O } ( 100 \operatorname { 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 .