Here is an fantastic podcast from PhD. Comics. The primary focus is on dark matter - on what we know (very little) and don't know (a lot). It is also a nice demonstration of how scientists talk about issues at the frontiers of science. The illustration works wonderfully here. This is only 6 minutes long, but it goes through a good deal of material.
Dark Matters from PHD Comics on Vimeo.
There is a a mention of Bullet Cluster towards the end. It is one of the stronger evidences for the existence of dark matter - still providing no clues about the nature of dark matter. If you are interested in the learning more about the Bullet Cluster, here is a short (5 min) Nova Now video focusing on this cluster: