  Observations show that for disk galaxies the fourth power of the circular
velocity of stars around the core of the galaxy is proportional to the
luminosity L. This is known as the Tully-Fisher law. Since L is proportional to
the mass M of the galaxy, it follows that the fourth power of the circular
velocity is proportional to M. Newtonian mechanics, however, tells that the
square of the circular velocity is proportional to M. In order to rectify this
big difference, astronomers assume the existence of halo dark matter. In this
paper we show that general relativity theory yields a term of the Tully-Fisher
form. This puts doubts about the necessity and existence of halo dark matter
for galaxies.
