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This interactive is used in Chapter 23. Look at telescope pictures of the Milky Way, and you’ll see countless stars, representing enormous mass. But there’s even more mass there that you can’t see! This Interactive will help you understand how astronomers know that. Set up a galaxy with no dark matter, and look at the speeds of the orbiting stars. Then increase the dark matter in steps, plotting the star velocities on the graph after each step. What happens to the velocities? Why? This Interactive will help you figure it out.