Studying spiral galaxies can make your head spin! With this video, find out how astronomers calculate a galaxy's rotational speed, and how the prediction that the outermost stars slow down does not seem to be true. The mystery may be solved by the presence of mysterious dark matter. Perhaps by showing this, you can inspire your space scientists to be the one who finally proves its existence! Because of the brevity of the clip, you may want to simply embed it within your own presentation.