  Within the framework of the Projective Unified Field Theory the distribution
of a dark matter gas around a central body is calculated. As a result the
well-known formulas of the Newtonian gravitational interaction are altered.
This dark matter effect leads to an additional radial force (towards the
center) in the equation of motion of a test body, being used for the
explanation of the so-called ``Pioneer effect'', measured in the solar system,
but without a convincing theoretical basis up to now. Further the relationship
of the occurring new force to the so-called ``fifth force'' is discussed.
