  The behaviour of the Dirac field in FRW space-time is investigated. The
relevant equations are solved to determine the particle and energy
distribution. The angular and radial parts are solved in terms of Jacobi
polynomials. The time dependence of the massive field is solved in terms of
known function only for the radiation filled flat space. WKB method is used for
approximate solution in general FRW space. Of the two independent solutions,
one is found to decay in time as the Universe expands, while the other solution
grows. This could be the source of the local particle current. The behaviour of
the particle number and energy density are also investigated. It is found that
the particles arrange themselves in a number and density distribution pattern
that produces a constant Newtonian potential as required for the flat rotation
curves of galaxies. Further, density contrast is found to grow with the
expansion.
