In the video below, you will see a good example of how Dark Matter keeps the galaxy as one, and prevents stars and planets from flying away. The main ideas too look out for in the video would be the right hand, and the string with a tennis ball attached to the edge. The string represents the galaxy; the tennis ball at the end of the string represents the mass of the universe, and finally, the right hand holding the sting, represents Dark Matter. When you watch the short video clip, you will see the hand holding the string and spinning it around. We use the spinning technique, because our galaxy today is spinning at all times. Picture this, if I was spinning, and I let go of the galaxy (the string) it would fly away. Well, that is exactly the purpose of Dark Matter.
