Astronomers model the growth of hierarchical structure in the universe using computer simulations. This simulation image maps the unseen dark matter belonging to the two colliding galaxy clusters. Gas is attracted by the gravity of the dark matter and heated as it flows into the higher-density regions, giving off X rays. Thus, X-ray images let astronomers trace the cluster's dark matter by mapping the high-energy emission from its hot gas.
The Virgo Consortium/Max-Planck-Institut für Astrophysik