Discovery of a hierarchical distribution of dark matter in the Fornax cluster of galaxies

THE mass of the Universe is widely believed to be dominated by dark matter(1). Dark matter is, by its very nature, extremely difficult to investigate, and the presence of dark matter is usually inferred indirectly through its gravitational influence on ordinary visible matter. Observations of X-ray-emitting gas associated with clusters of galaxies can help to constrain the large-scale distribution of dark matter; if the gas is in hydrostatic equilibrium with the gravitational potential of the cluster, it will trace the distribution of all matter present (dark and visible)(2). Here we present X-ray observations of gas in the Fornax cluster of galaxies, which show that dark matter is distributed on at least two distinct length scales: the scale of the dominant central galaxy and that of the cluster as a whole. This suggests the presence of either a single form of dark matter exhibiting hierarchical clustering (analogous to the hierarchical distribution of visible matter), or two forms of dark matter which interact-and hence cluster-through different unknown mechanisms.