Seeing Dark Matter..
I found this intruiging and impressive image over at Cosmic Variance (there’s also a press release at the Hubble Space Telescope website with higher resolution images). It shows the giant cluster of galaxies Abell 1689 with, superimposed on it, a map of the matter distribution as reconstructed from the pattern of distortions of background galaxy images caused by gravitational lensing.
This picture confirms the existence of large amounts of dark matter in the cluster – the mass distribution causing lensing quite different from what you can see in the luminous matter – but it also poses a problem, in that the matter is much more concentrated in the centre of the cluster than current theoretical ideas seem to suggest it should be…
You can find the full paper here.