 | {\it Left panel:} Image of the cluster Abell 2218 taken with the Hubble space telescope (see Ref.~\cite{Kneib:1995hh}). Spectacular arcs resulting from strong lensing of background galaxies by dark matter in the cluster are clearly seen. {\it Right panel:} Deep Chandra image of the Bullet cluster. The X-ray brightness of the gas component is coded in yellow, red and blue colours. Distribution of the gravitating mass, obtained from weak lensing reconstruction, is shown by green contours, from~Ref.~\cite{Clowe:2006eq}. |