  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.
