I'm trying to create a kind of mystical shadow thing, made from black particles of varying size. Essentially, I'm using a shape (I've tried it with several things: A cube, a custom shape I made, a cylinder, a sphere, etc) as the outline, and trying to have a bunch of particles surrounding the edge of the shape, or else filling in the inside without going out, or much out.
I can get it to work for a still image by just having the start and end frames set to 1, but for an animation, how would I get them to constantly move around (not just bounce off the walls like when I tried having an invisible collision shape) inside the volume of the object?