Alternatives to the cold , collisionless dark matter ( DM ) paradigm in which DM behaves as a collisional fluid generically suppress small-scale structure .
Herein we use the observed population of Milky Way ( MW ) satellite galaxies to constrain the collisional nature of DM , focusing on DM–baryon scattering .
We first derive conservative analytic upper limits on the velocity-independent DM–baryon scattering cross section by translating the upper bound on the lowest mass of halos inferred to host satellites into a characteristic cutoff scale in the linear matter power spectrum .
We then confirm and improve these results through a detailed probabilistic inference of the MW satellite population that marginalizes over relevant astrophysical uncertainties .
This yields 95 \% confidence upper limits on the DM–baryon scattering cross section of 6 \times 10 ^ { -30 } cm ^ { 2 } ( 10 ^ { -27 } cm ^ { 2 } ) for DM particle masses m _ { \chi } of 10 keV ( 10 GeV ) ; these limits scale as m _ { \chi } ^ { 1 / 4 } for m _ { \chi } \ll 1 GeV and m _ { \chi } for m _ { \chi } \gg 1 GeV .
This analysis improves upon cosmological bounds derived from cosmic-microwave-background anisotropy measurements by more than three orders of magnitude over a wide range of DM masses , excluding regions of parameter space previously unexplored by other methods , including direct-detection experiments .
Our work reveals a mapping between DM–baryon scattering and other alternative DM models , and we discuss the implications of our results for warm and fuzzy DM scenarios .