You mean that this process has the appearance of decreasing entropy? In truth it doesn't. For example gravitational collapse (the basic mechanism of galaxy and star formation) decreases entropy by reducing the spatial spread of matter but increases entropy by heating matter up. Thus we end up with a total entropy gain. On cosmic scale, I think the process is exploiting a sort-of temperature difference between gravity and matter, namely that initially the temperature of matter was much higher than the Unruh temperature associated with the cosmological constant. Thus even though the initial state had little structure it was very off-equilibrium and thus very low entropy compared to the final equilibrium it will reach.