Scientists now think they know how fast dark matter would fall if it were dropped on other dark matter. Like the dark matter version of Galileo's experiment of dropping items to the ground from the tower of Pisa, theorists from the California Institute of Technology and the Canadian Institute for Theoretical Astrophysics in Toronto found that there is no more attractive force associated with dark matter than with normal matter. This finding helps to eliminate other theoretical models created to explain phenomena that required an extra force within dark matter. The results of the work are presented in the September 28th 2006 issue of Physical Review Letters.
Dark matter has long fascinated scientists ever since its effects were detected in the 1970s. It is matter that does not emit or reflect light and so cannot be detected directly, but it is understood to exist from its gravitational effects on visible matter, namely its effect on the speed of stars around galaxies. Scientists still don't know what makes up dark matter. 
The work done was mostly theoretical, but the program was written to simulate observations made of the Sagittarius galaxy. The effect the team was looking for would be caused by tidal forces between the galaxy's dark matter and the Milky Way's dark matter. Just like the moon exerts a pull on the earth, causing tides, so does the Milky Way exert a force on the Sagittarius galaxy. The scientists were able to conclude that the dark matter affect other dark matter in the same way regular matter does. Kesden and Kamionkowski, the co-author from the California Institute of Technology, tried more than one distribution of the dark matter but found the current theory fits best with the data. This theory says that most dark matter around a galaxy is located around a galaxy in a kind of halo, and is made of lots of small particles.
The prediction that there may be an extra force associated with dark matter appeared in part to explain strange phenomena in the universe. One such phenomenon is the universal structure of galaxies. Galaxies tend to clump themselves together, with voids in between. Computer simulations showed that these voids contained dark matter. It had been theorized that this occurred because there was an extra force in dark matter clumping it together. Kesden mentions that these structures may be caused by other forces that affect only regular matter, like electricity and magnetism, but not dark matter. These forces would clump normal matter together and not affect the dark matter. They now conclude that the force is not gravitational.
Scientists, such as Glennys Farrar, a theoretical physicist at New York University, who examined the paper's arguments, said that the researchers made too many assumptions about their simulated dark matter.  Kamionskowski responded with, "We know pretty well where the dark matter is in the Milky Way." Therefore they could extrapolate that for other galaxies. However, he continued to mention that other scientists might want a more detailed comparison of the simulation with the data. Kesden states that there will always be doubters. He says a researcher then has to think, "What kind of tricks can you play to hide such an [extra force]? So it's a game. You have to be sneaky to rule everything out." As far as his game goes, "Ours was just for a typical scenario, assuming there wasn't a conspiracy to hide the force." 
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Dark matter lacks extra gravitational force