  Bars and spiral arms have played an important role as constraints on the
dynamics and on the distribution of dark matter in the optical parts of disk
galaxies. Dynamics linked to the dissipative nature of gas, and its
transformation into stars provide clues that spiral galaxies are driven by
dissipation close to a state of \textit{marginal stability} with respect to the
dynamics in the galaxy plane. Here we present numerical evidences that warps
play a similar role but in the transverse direction. N-body simulations show
that typical galactic disks are also marginally stable with respect to a
bending instability leading to typical observed warps. The frequent occurrence
of warps and asymmetries in the outer galactic disks give therefore, like bars
in the inner disks, new constraints on the dark matter, but this time in the
outer disks. If disks are marginally stable with respect to bending
instabilities, our models suggest that the mass within the HI disks must be a
multiple of the detected HI and stars, i.e., disks must be heavier than seen.
But the models do not rule out a traditional thick halo with a mass within the
HI disk radius similar to the total disk mass.
