I always love these Hubble deep space images, but this one is especially cool because of what you can't see.
Galaxy cluster Cl 0024+17 is in the center of this image. Around it you can see background galaxies that appear distorted because of the strong gravitational field, something that you often see in similar images. By mapping the distortion of the light, astronomers were able to spot a ring of dark matter around the cluster. We can't see dark matter, of course (and don't really know what it is) but here its effect on the visible universe lets us know it's there.
Image Credit: NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University)