> we don't have an explanation of why those distributions of DM are balls around galaxies and not balls in the intergalactic medium We do. Structure formatio

> we don't have an explanation of why those distributions of DM are balls around galaxies and not balls in the intergalactic medium We do. Structure formation is part of the standard cosmology. https://www.esa.int/Science_Exploration/Space_Science/Planck/History_of_cosmic_structure_formation https://www.esa.int/Science_Exploration/Space_Science/Planck... https://kipac.stanford.edu/research/cosmic-structure https://kipac.stanford.edu/research/cosmic-structure etc etc. Matter fell into place wherever it could, forming overdense areas. More matter tended to fall into overdense areas from underdense areas, and this allowed the underdense areas to expand more easily while the overdense areas were constrained to expand more slowly or even to collapse. Dark matter, not being able to collide and radiate away the collision-energy collapses much more slowly than heavy matter. Dark matter connects fluctuations in the cosmic microwave background with large scale structures -- the cosmic web -- extremely well, as long as it moves nonrelativistically (that's why it's "cold" compared to electron neutrinos which move ultrarelativistically) and collapses very slowly. > ultradense ball of lambda-CDM Mu? Lambda (capital greek letter) is the term that encodes the expansion of space and the consequent separation of clusters of galaxies from one another. It's irrelevant within clusters of galaxies, let alone individual galaxies. The solar system is not expanding at all, and neither is Manhattan. Lambda is also a constant. It doesn't form balls, there are no overdensities or underdensities, it has the same tiny value at every point in spacetime. CDM is cold dark matter, and your phys.org link discusses evidence for a possible explanation about why there isn't a high density "ball" of dark matter in the galactic centre compared to around our solar system: maybe dark matter can interact electromagnetically under conditions found in galactic cores. The electromagnetic interaction in this study would produce a gamma ray and the vanishing of a dark matter particle. This could be some complicated way dark matter can self-annihilate in the presence of very hot standard model gas like that around black holes or produced by supernovae, for instance. Whatever the specifics, this conversion of dark matter to gammas would keep the central dark matter relatively sparse. (Other mechanisms for avoiding high dark matter densities in galaxy cores are available. These do have a good collective name: galactic outflow, of which dark matter heating and dark matter decay are subcategories that are at the root of your phys.org link. Generally dark matter turns into hot matter, or drags hot matter along with it for a ride. Outflow must enrich the metallicity of stars far from galactic cores, and quench core star formation, and these generate observables for individual proposed mechanisms, and astronomers are already happily gathering up data that probe those observables. Good proposals for outflow mechanisms also work on the scale of structures like the Coma Cluster).