It may not be immediately obvious the first time you see this image, but notice the ring of slightly darker color around the center. That's a simulated map of the dark matter halo around galaxy cluster Cl 0024+17, superimposed on a Hubble picture. The ring may have been formed when two galactic clusters collided.
Images like this are one of the ways researchers detect dark matter through its effects. In this case, Hubble observed how the gravity of this cluster distorted the light from more distant galaxies, and determined that the cluster's ordinary matter couldn't account for all of the distortion.