  Based on N-body simulations, we show that realistic galactic disks are
subject to bending instabilities of fire-hose type when the disks are
substantially self-gravitating, that is, if they contain dark matter
distributed in the disk. Depending on the degree of instability, S and
U-shaped, as well as asymmetric warps are generated. In some cases, the warp
may last several galactic rotations, particularly when the instability is
marginal. Since the bending instability is very sensitive to the disk
flattening, the fractions of dark matter distributed in the disk and in the
dark halo are constrained. For a Milky Way like galaxy the extended dark halo
can not exceed 30-40% of the total mass within 35 kpc if the Milky Way warp
results from a bending instability. This mode of warping provides a unified
picture of spiral galaxies, where bars, spiral arms and warps result all from
disk gravitational instabilities, radial or transverse, which are constantly
regenerated by the dissipative gas component.
