I have often tried to simulate the rotation of a Galaxy in Algodoo but always failed, today I succeeded. As you move out from the centre the rotation speed should increase but when you get to the edge it should decrease (without dark matter). This is because as you move away from the centre there is more and more mass to orbit around until you get to the edge. In Algodoo you can't get the particles to orbit the other particles, I faked it here by hiding 3 masses at the centre. By setting the interaction layers the innermost star is only attracted to the inner mass, the next to a bigger mass and the next to an even bigger one. The outer stars are attracted to all the masses.

What's the point in faking it? Solving the problem helps you to understand the real situation.