Fluid Dark Matter
  Dark matter modeled as a classical scalar field that interacts only with gravity and with itself by a potential that is close to quartic at large field values and approaches a quadratic form when the field is small would be gravitationally produced by inflation and at the present epoch could act like an ideal fluid with pressure that is a function only of the mass density. This could have observationally interesting effects on the core radii and solid body rotation of dark matter halos and on the low mass end of the primeval mass fluctuation power spectrum. 
P. J. E. Peebles