Every once in a while, we encounter pictures which give us pause and put our lives in perspective. This one definitely made me stop and wonder. As described on HubbleSite, this photograph demonstrates the illusive concept of dark matter, a substance that cannot be seen, but that is thought to "hold together galaxy clusters." If this isn't enough to inspire, the white "dots" on this picture are actually entire galaxies! If you visit the site, you can actually zoom in and scan the picture. It is truly awesome.
Some other sites of interest
Top Ten Astronomy Pictures of 2007
ESA Space Images