Galactic warps represent an old unresolved problem since the discovery of the Hi warp of the Milky Way at the end of the fifties. In this paper, we propose a new scenario explaining a large fraction of the observed optical warps. Based on N-body simulations, we show that realistic galactic disks, where the dark matter is essentially distributed in a disk, are subject to bending instabilities. S, U-shaped, as well as asymmetric warps are spontaneously generated and in some cases are long-lived. While this scenario presents the advantage of explaining the three observed types of warps, it also brings new constraints on the dark matter distribution in spiral galaxies. Finally, it gives us a unified picture of galaxies, where galactic asymmetries like bars, spirals, and warps result from gravitational instabilities.