I stumbled across a wonderful 90-minute video on youtube that was originally produced by National Geographic as a two-hour program for their cable channel, titled "The Definitive Guide to the Milky Way Galaxy"
that provides a richly graphic, yet quite substantive condensed astronomy course. One of the most fascinating aspects of the video are the hypothetical clips of what earth's nighttime sky might look like from a particular coastal location if: a) earth was within a globular cluster;
we were in the future epoch where Andromeda and the Milky Way galaxy were nearing collision; c) the solar system was nearer the center of the galaxy. The video contains one of the most succinct, straightforwardly convincing explanations (graphical and verbal) for why astrophysicists are convinced "dark matter" exists that I've ever come across. The graphics (provided with help of JPL and NASA) are quite excellent, all except for the occasional repetition of a misplaced and awkward amusement-park ride metaphor, a needless instance of dumbing-down what was otherwise a solid presentation aimed at a sophisticated, intelligent lay audience. But the other 95% is so excellent that it's worth forgiving some NG producer for inserting this dubious element into an otherwise fascinating graphical presentation.