Algodoo

Here, many square particles can fly around a Black Hole and their gravities will merge eachother. The particles are frictionless, and square shaped so they can slide against eachother easily, simulating fluid dynamics like planets are in reality. The borders are to bounce lost matter back to the center, for re-use.
When an object is eaten by the singularity, it gets spawned back in orbit, and loses a bit of mass, again for re use to simulate accretion. The black hole simulates graviotational delay with a very thin atmosphere that will overtime slow the particles, causing them to fall into the black hole to prevent one body from gaining 100% of the matter. This will keep things fresh. Hope this is well! Comment any suggestions.