According to the standard model of cosmology, each galaxy contains huge amounts of dark matter, which can only be detected thanks to its gravitational pull. Without it, galaxies would be torn apart, with their stars flying away from each other. Dark matter’s nature remains a mystery, a mystery that has just grown bigger…

The standard model describes the Universe as being mainly made of dark energy and dark matter (I’d like to remind you that there are also various alternatives, at least as interesting). The vast majority of astronomers agree on the fact that dark matter is “cold” (moving slowly), and that over time clumps of dark matter are formed  and growing, attracting “normal” matter and forming galaxies.

Many simulations established that the density of dark matter is much higher in the centers of galaxies… But a new study just showed that a particular kind of galaxies – dwarf galaxies – have a rather homogeneous distribution of dark matter. In other words, this suggests that the standard model may be wrong.

Unless astronomers fiddle with the model, predictions don’t agree with observations…

Dwarf galaxies are mainly made of dark matter (up to 99% of their mass), the rest being stars. This feature makes dwarf galaxies the perfect candidates to study and try to better understand dark matter.

Matthew Walker from Harvard-Smithsonian Center of Astrophysics, and Jorge Peñarrubia from University of Cambridge analyzed the dark matter distribution in two Milky Way neighbors: the Fornax and Sculptor dwarf galaxies. They hold one to ten million stars (our Milky Way holds 200 to 400 billion stars). The two researchers measured the locations, speeds and chemical compositions of 1,500 to 2,500 stars.

According to their estimates, in both cases dark matter is uniformly distributed over large regions, several hundred light-years wide. This contradicts the prediction that the density of dark matter should significantly increase in the centers of these galaxies.

Some have suggested that interactions between matter and dark matter could have spread out the dark matter, but simulations haven’t shown this kind of phenomenon in dwarf galaxies. Thus, this new measurement shows that either there are some unexpected interactions between matter and dark matter, or that dark matter isn’t “cold”. Finally, this could also imply that there is no such thing as dark matter, and that an alternate explanation is necessary… Hopefully, the two scientists may find an answer by studying more dwarf galaxies, particularly ones containing even more dark matter.



Matthew G. Walker, Jorge Peñarrubia. A Method for Measuring (Slopes of) the Mass Profiles of Dwarf Spheroidal GalaxiesarXiv:1108.2404v3