Scientists now believe that interactions between dark and ordinary matter is more important and complex than previously thought. For instance, dark matter 'knows' how the visible matter is distributed and they conspire with one another such hat gravity of visible matter at the characteristic radius of the dark halo is always the same. Therefore, it is possible that a non-gravitational fifth force is ruling dark matter, leaving the same mark on all galaxies irrespective of their ages, shapes and sizes. This might be an explanation for the mystery of 'dark energy'. This research is important to the theories that relate to the history and expansion of the universe.
If we account for our observations with a modified law of gravity, it makes perfect sense to replace the effective action of hypothetical dark matter with a force closely related to the distribution of visible matter.
Reference:
Gianfranco Gentile, Benoit Famaey, HongSheng Zhao, Paolo Salucci. Universality of galactic surface densities within one dark halo scale-length. Nature, 2009; 461 (7264): 627 DOI: 10.1038/nature08437
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