Just dust : About the (in)applicability of rotating dust solutions as   realistic galaxy models
  Solutions of the stationary axisymmetric Einstein equations describing the interior of circularly rotating dust are investigated in order to study their potential applicability as galaxy models. It is shown that such interior solutions cannot be used as global metrics without becoming unphysical in certain regions of space. Although definite results concerning the non-existence of a continuation into a vacuum can only be derived for interior solutions describing rigidly rotating dust, the present analysis exhibits that the case of non-rigidly rotating dust would in general also be inadequate as a physically reasonable galaxy model. 
Tobias Zingg, Andreas Aste, Dirk Trautmann