Evanescent Matter
  The LCDM cosmology offers a picture for galaxy formation that is broadly promising but difficult to reconcile with the evidence that environment has had strikingly little effect on the evolution of ellipticals and pure disk spiral galaxies. Reconciliation might be aided by adding to LCDM an evanescent component of matter with evolving mass and a fifth force large enough to aid early assembly of more nearly isolated protogalaxies. I present a simple illustration. 
P. J. E. Peebles
astro-ph.CO