A Brief Explanation of What is Happening
Each of these particles is a point mass in a vacuum, they all exert a gravitational pull on each other. All of the starting positions are random, so it will take some time for the system to come to a stable state. There are some floating point issues, but otherwise the system conserves energy, and will continue to move permanently.
It won't always happen, but I find it really cool that this simple simulation will still get orbiting bodies occasionally. If you give it a few tries and look closely you might be able to find one.
This system of 3d is something I started building out a long time ago, and that I discuss more in my post about making 3d graphics in raw JS