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A force acting only on dark matter (DM) cannot be probed in the lab. Yet when it is attractive, and acts coherently across astrophysical distances, it can dominate over gravity even at very weak coupling. Such a force is tightly constrained cosmologically and in the Bullet Cluster, unless its range is sub-galactic, i.e., at or below a kiloparsec. In this talk I describe its effects on dwarf galaxies. The DM would feel a deeper potential in these objects, while stars would continue to respond only to gravity. For degenerate fermions, I show that the attraction can therefore balance a greater Fermi pressure than gravity alone, allowing for eV-scale constituent masses. This is a factor of 100 below the usual Tremaine-Gunn limit, imposed by the maximum phase-space occupation within these structures. The same equilibrium fixes a mass-radius relation for the resulting cores, and should also shape the structure of larger halos such as the Milky Way (despite the force being screened at those scales).

Next I explore its effects on cosmological history, which set the strongest requirements on this scenario. Perturbations collapse rapidly below the force range, ultimately forming stable bound states of that size and with roughly the ambient density. To reproduce the structures observed at kpc scales, the attraction must therefore remain inactive until the mean density drops below that of dwarf galaxies. This corresponds to z~100, after recombination but before the first stars. The force therefore leaves the CMB untouched while enhancing power at high wavenumbers, with potential baryonic signatures at cosmic dawn. I conclude by presenting a concrete particle model to realize the required delayed onset: a phase transition triggered when cosmic expansion lowers the DM density below a fixed threshold.