When you see this image in the popular media, be sure to realize it's a statistical map of the distribution of dark matter around a galaxy cluster _superimposed_ on an image of the cluster. It's not an image of dark matter. The actual image is this one:
By looking at the blue arcs in the image, astronomers can map how the massive galaxy cluster bends light from a more distant galaxy behind the cluster, and compare that distribution with what they see in the visible.
Here's a crop of the upper-left section of the cluster--you can see distorted images of a blue, star-forming, possibly spiral galaxy. The paper will probably have a reconstituted image of the lensed galaxy.
It's unfortunate that many people will see the blue image and think Hubble directly imaged some dark matter. The more important detail of the science was the ring shape and the proposed physics behind the shape from galaxy interactions.
More discussionn at Bad Astronomy
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