The concentrations of the cuspy dark matter halos predicted by simulations of cold dark matter are related to the cosmology in which the halos form .
Observational constraints on halo concentration therefore map into constraints on cosmological parameters .
In order to explain the observed concentrations of dark matter dominated low surface brightness galaxies , we require a cosmology with rather little power on galaxy scales .
Formally , we require \sigma _ { 8 } \Gamma _ { 0.6 } < 0.23 , where \Gamma _ { 0.6 } is a modified shape parameter appropriate to this problem .
Practically , this means either \Omega _ { m } < 0.2 or \sigma _ { 8 } < 0.8 .
These limits apply as long as we insist that the cuspy halos found in simulations must describe the halos of low surface brightness galaxies .
A low density cosmology helps with the low observed concentrations , but it offers no explanation of the many cases where the shape of the density profile deviates from the predicted cuspy form .
These cases must have suffered very extensive mass redistribution if the current halo formation picture is not to fail outright .
It is far from clear whether any of the mass redistribution mechanisms which have been suggested ( e.g. , feedback ) are viable .