Warm gas in the outskirts of galaxy clusters -- The cluster soft excess   phenomenon
  The cluster soft excess emission indicates the presence of large amounts of warm gas (T ~ 10^6 K) in the neighborhood of galaxy clusters. Among the clusters that display this phenomenon is the Coma cluster, the nearest rich galaxy cluster. The excess emission is more prominent at the cluster's outskirts than at its center. Detailed studies of its large-scale emission -- up to ~2.6 Mpc from the cluster's center -- reveal that these warm baryons are as massive as, or possibly more massive than, the well-known hot intra-cluster medium (T ~10^8 K). A possible interpretation of the excess emission from the Coma cluster is radiation from low-density filaments located in the neighborhood of the cluster. In this case, the filaments would extend for much larger distances, or feature higher density, than predicted by current cosmological simulations. 
M. Bonamente, R. Lieu and J.P.D. Mittaz