Spiral galaxies and their dark halos are self-organized patterns.

Motivated by and generalizing the Cross-Newell energy functional for pattern-forming systems, we propose a modification of the Einstein-Hilbert action on galactic scales. We introduce additional terms that reflect the presence of patterns, like fingerprints, with the same length scales as the clumping instabilities of baryonic matter. The resulting pattern structures carry an additional energy density which, remarkably, accounts for many of the phenomena that are generally attributed to dark matter.