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.
Peebles
PJ