  We explore a model for a fermionic dark matter particle family which
decouples from the rest of the partices when at least all standard model
particles are in equilibrium. We calculate the allowed ranges for mass and
chemical potential to be compatible with big bang nucleosynthesis (BBN)
calculations and WMAP-data for a flat universe with dark energy. Futhermore we
estimate the free streaming length for fermions and antifermions to allow
comparison to large scale structure data (LLS). We find that for dark matter
decoupling when all standard model particles are present even the least
restrictive combined BBN calculation and WMAP results allow us to constrain the
initial dark matter chemical potential to a highest value of 6.3 times the dark
matter temperature. In this case the resulting mass range is at most 1.8 eV < m
< 53 eV, where the upper bound scales linearly with the effective degrees of
freedom at decoupling. From LSS we find that similar to ordinary warm dark
matter models the particle mass has to be larger than approximately 500 eV
(meaning the effective degrees of freedom at decoupling have to be > 1000) to
be compatible with observations of the Ly alpha forest at high redshift, but
still the dark matter chemical potential over temperature ratio can exceed
unity.
