In my earlier post Entropy and Universe , I had stated that entropy of universe always increase as there are more possible number of disordered arrangement than ordered arrangement. After the big bang, gases and dust like matter started to spread throughout the universe. It is like opening the cap of your favourite coke bottle and letting the gas leak and randomly distributing in its surrounding. This increases overall entropy of universe (refer Entropy and Universe).
In this case, one thing we had completely ignored is the effect of gravity, which is a right approach as in this case gravity hardly makes a difference. But when we reach a larger domain like universe, gravity becomes fundamental and hence it cannot be ignored. As the gases spread throughout universe, each particle attracts the other and congregate to form larger clumps like stars, nebulae etc. Now clumps means particles are more organized and in ordered state, but entropy means randomness. So does that mean entropy decrease after universe banged. The answer is no. This is because as particles got together, large amount of energy is released as heat which means an increase in entropy.
In formation of ordered state there is more than enough compensating disorder. This is second law of thermodynamics. And this is the sole purpose of gravity, to increase overall entropy by creating ordered state like stars, galaxies etc.
Gravity is the one and only force yet known which causes entropy at macroscopic domain to increase. This is because it exist between every particle and its range is infinite. When universe was formed, matter was homogeneously distributed. With gravity, black holes were formed which concentrated matter into a singularity, and thus creating a heterogeneous universe. In fact black holes are the most heterogeneous entity found in our universe. Even though Black holes emit radiations known as Hawking radiation, still they have highest entropy. This is due to fact that it does not let light to escape from its gravitational pull (refer The Black Hole: Facts and Basics) and limits our observation beyond event horizon. So there can be infinite possible random arrangements of anything in its range. And more randomness means high entropy.