In the TGD framework (see this, this, and this), the rotation velocity is proportional to the square root of the product GT, where T is the string tension of a long magnetic flux tube formed from a cosmic string carrying dark energy and possibly also matter. No dark matter halo is needed. In the ordinary situation, the flux tube would be considerably thickened only in a tangle associated with the galaxy as part of volume- and magnetic energies would have decayed to ordinary matter, in analogy with the decay of the inflaton field.
Whereas the dark halo model predicts that any rotation plane for the distant stars is possible, TGD allows only the galactic plane orthogonal to the long cosmic string, and distant stars move in the general case along helical orbits along the cosmic string.
The TGD view also allows us to understand why MOND works in some cases: in the TGD framework the critical acceleration acr, suggested originally to be constant of Nature, is replaced with string tension.
If the flux tube itself has a very long thickened portion such that ordinary matter has left this region by free helical motion along the string or by gravitational attraction of some other object, the string tension T is small and very small rotation velocity is possible. Ordinary bound states of matter are not necessary since the gravitational force of the flux tubes binds the stars to the flux tube. This might explain why the galaxy can be ultradiffuse.
For a summary of earlier postings see Latest progress in TGD.