Algodoo

Indense is the name of a group, the group containing both the red and green particles in this scene. Particles of one color don't interact in any direct way, but particles of opposing colors attract together. This has the side effect of allowing an arbitrarily large amount of particles of the same color to be in the same location. Also, having two different types of particles which don't interact with their own type means the simulation runs much faster than if every particle interacted with every other one. It looks a lot more like galactic simulations than anything i've ever created, except for the part where things explode too violently.