Can dark matter be mostly massless particles?
  We discuss an overlooked factor in dark matter studies. Namely, if massless particles are captured into a local structure and stop free streaming in the universe, they no longer lose energy by cosmological red-shift, and no longer smear out density fluctuations beyond their ``confinement'' scale. If this occurred at the stage when radiation dominated over baryonic matter in energy density, then these captured massless particles would comprise the major part of dark matter in today's universe, leaving no room for other dark matter scenarios. The most probable such particles are gravitons with non-linear self-interaction. 
Xiang-Song Chen