How do you visualize something that can't be seen? By visualizing the effect that it has on the stuff that can be seen. This is a web application that helps us to understand how dark matter (which is more appropriately named transparent matter) influences gravity in a galaxy.
This was developed for the visualization subcategory and is currently hosted at: http://homepage.cs.uiowa.edu/~stolenti/transparent-matter/index.html.
Demo from Hack the Universe presentations: http://youtu.be/npBUxnFUkm4?list=UUIuhq9LTleLC-GMdAOvvZcg