 Just how the universe evolved, from small-scale matter deviations at the time of decoupling, to filaments of superclusters and vast voids, can be explained by a physical process called caustics. Originally developed to explain light behavior, it works just as well for protons and dark matter. Picture a set of uniformly distributed particles on a line, each with slightly different velocities. They start out with a uniform particle density, but because of the small velocity differences, the particle density will vary as time goes by, areas of high and low density will develop. The density at a later time, t, is described by an equation. The equation has hotspots when the denominator approaches zero. There's a plot of the rate of change in velocity with respect to location on the x-axis times time. Naturally we get a very small curve when time is small, but as time goes on, the deviations grow. When they reach the critical time and beyond, we get hotspots. Expanding this to two dimensions, we get density peaks along curved lines that themselves intersect at points with maximum intensity. I see this phenomena in my own backyard. The lines at the bottom of a swimming pool are examples of caustics caused by small waves on the surface of the water. And when we extend this to three dimensions, we get curved surfaces with increased density that intersect along lines that intersect at points. This is the web-like pattern we see in the large-scale universe. This is what you get from the initial conditions characterized by the cosmic microwave background. No additional forces are involved. Gravity is the key to the formation of stars and galaxies within these filaments of dense accumulations of baryons in dark matter. But it was caustics that took the minor anisotrophies that existed at the time of decoupling and turned them into giant walls around great voids.