The Kelvin-Helmholtz Instability in an Expanding Universe and its Effect   on Dark Matter
  We extend the Kelvin-Helmholtz instability to an expanding background. We study the evolution of a non-viscous irrotational fluid and find that for wavelengths much smaller than the Hubble scale small perturbations of the fluid are unstable for wavenumbers larger than a critical value. We then apply this result in the early universe, treating cold dark matter as a classical fluid with vanishing background pressure. 
Karim A. Malik (IAP) and David R. Matravers (ICG)