Galaxy

The universe is fractal, to some degree.  Just as the Mandelbrot Set keeps repeating at ever smaller or larger scales, always redefining the same pattern, so does the world.  But the Mandelbrot Set does not repeat exactly at different scales, but always has a new variation on the same pattern.

In the real world, in a similar way,the same pattern keeps repeating at different scales.  When atoms were first observed people thought they would be like little solar systems, and they are a little bit, with a central power and cirulating wave/particles, but they are not exactly the same but smaller, but have a variation.  Similarly at a greater scale, galaxies have the stars instead of planets or electrons, and a great black hole in the centre instead of the sun or the nucleus.  These three levels appear to be the only ones, as larger than galaxies appears to be more like a foam and smaller than the atoms becomes apparently random, again without distinguishing structure at any greater or lesser scale.