Sorry I forgot to mention that it these shadows are created by the so called Sunyaev-Zel'dovich effect.

Sunyaev-Zel'dovich effect: One of the most exciting and promising new ways of measuring Galaxy cluster masses is through their imprint on the cosmic microwave background (CMB, or as "real cosmologists" call it, the "camb"). As a CMB photon goes through a cluster, it will interact with some of the high energy electrons in the cluster's hot gas (the same ones responsible for the X-ray emission). The result is that the CMB photon gains a little bit of energy, causing the CMB to appear hotter in the direction of the cluster than it would be in the cluster's absence.