This question came up in a family discussion recently: if dark matter is, well, dark and current theory says it doesn't interact with normal matter, then how can it actually be detected? And the answer is that the dark matter isn't detected -- it is inferred from other obser- vations. This photo [Credit: NASA/ESA, M. J. Jee & H. Ford et al. (Johns Hopkins University)] of the galaxy cluster CL0024+17 in the constellation Pisces shows, in addition to galaxies that are part of the cluster, a series of repeated images of background galaxies that were created by gravitational lensing.
This lensing effect is caused by light being bent by travelling through a region where there is strong gravity before reaching us. Some analogies can be set up using everyday objects, such as looking at distant streetlights through a wine glass.
In the case of the cluster in the photograph, the lensing does not occur immediately around the edge of the cluster itself, as one might expect, but in a ring some distance away from the centre of the cluster itself. The lensing (bending of light) is being done by the gravitation associated with unseen mass. Based on the amount of lensing, a computer model was developed to determine the location of the unseen mass, which has been coloured blue in the photograph.
Here is a video which explains further, including some discussion on the origin of the ring (which at 2.5 million light years across is comparable to the distance from the Milky Way to Andromeda):