The minutephysics Channel on youtube has a short but great example for creating a simulation of the Universe using After Effects with an image or fractal noise, and using the standard Particle Playground effect in After Effects.

The Simulation uses some simple concepts by an image or a Fractal Noise Effect in After Effects to drive particle mass where particles that are darker will attract more, and lighter particles act as if they are less dense. In the end, the Colorama effect is used to remap the particles from black and white to something that has some color.