Considering that dark matter is supposed to make up about 23% of all mass-energy density in the universe, it’s surprisingly difficult to pin down. It can’t be seen or measured directly, that much is known. Its existence is inferred from gravitational effects on things that instruments can see and from gravitational lensing (the bending of light as it passes through galaxies).
One of the prevailing inferences, generated by computer models, is that dark matter should clump together toward the center of galaxies. This clumping allows the material in dark matter to condense into visible matter, eventually forming stars and galaxies that we see with our eyes and telescopes. This behavior is obviously fundamental to the notion of how the universe forms, its cosmology.
A good place to look for the effects of dark matter is in so called small galaxies. These are galaxies with only a few million stars; a normal galaxy has hundreds of billions of stars. With so few stars, it is estimated that small galaxies are about 99 percent dark matter, which should increase the probability of detecting the effects of dark matter. A new study of small galaxies by Matthew G. Walker and Jorge Peñarrubia at the Harvard-Smithsonian Center for Astrophysics (Cambridge, Massachusetts, USA) to be published in The Astrophysical Journal and available online [arXiv [11 August 2011 A Method for Measuring (Slopes of) the Mass Profiles of Dwarf Spheroidal Galaxies] looked at the distribution of matter in two ‘neighboring’ small galaxies of the Milky Way, Fornax and Sculptor.
What they found was surprising: the dark matter appears to be evenly distributed across several hundred million light years in each galaxy. This contradicts the prediction of dark matter being concentrated toward the center. If the finding holds, it will require major revision in many of the assumptions of astrophysics, especially on the nature of dark matter and its interaction with normal matter. Either normal matter affects dark matter more than is currently thought or dark matter is not ‘cold’ – without energy, passive and inert.
This is now a good piece of scientific ‘mystery’ with hypotheses that can be tested and validated. There is always the possibility that with such a small sample, only two galaxies, that error in measurement or analysis are at work. In short, many more small galaxies need to be studied and the distribution of dark matter refined and validated. This involves many years of work and should be done by scientists other than the original team as well. However, the finding is challenging. If it holds up, or if no satisfactory explanation is devised, it will have major impact on the theories of dark matter.