  Many independent high resolution simulations have indicated that the standard
collisionless cold dark matter model does not reproduce the structure of
observed present day galaxies well. Several possible solutions in the form of
modifications to the physics of the dark matter particles have been proposed.
One of the most promising is warm dark matter (WDM), particles with significant
thermal motion in the early universe. It is usually assumed that such particles
are relativistically decoupled particles with a mass of approximately 1 keV.
However, here we have investigated the possibility that much more massive
particles with highly non-thermal spectra could make up warm dark matter.
Several possible production mechanisms are reviewed and the only one found to
be viable is that the WDM is produced by the non-relativistic decay of some
massive species in the early universe. Such very massive warm dark matter could
possibly be detected in direct detection experiments, as opposed to standard
thermal warm dark matter.
