Bullock et al (2001) demonstrated that dark halos have a
universal angular momentum distribution that should also be
characteristic for the infalling gas component. Van den Bosch et
al. (2001) lateron showed that this angular momentum
distribution is not consistent with the observed distribution of
exponential galactic disks indicating that viscous angular
momentum redistribution in galactic disks must have played an
important role. The viscosity is likely driven by interstellar
turbulence which is a result of stellar energetic feedback
processes (see Fig. 2) or global disk instabilities (magneto-
rotational instability or gravitational instability). Note, that
viscous effects will increases the angular momentum problem
substantially as viscosity in general removes angular momentum
from the dominante mass component in the disk and transfers it
to the outermost parts of the disk. Figure 2. Gas surface
density of a gravitationally unstable gas-rich galactic disk,
embedded in a dark matter halo. The left panel sh