People say there was a primordial explosion, an enormous bang billions of years ago which flung all the galaxies into space. Well let's take that just for the sake of argument and say that was the way it happened. It's like you took a bottle of ink and you threw it at a wall. Smash! And all that ink spread. And in the middle, it's dense, isn't it? And as it gets out on the edge, the little droplets get finer and finer and make more complicated patterns, see?