If gravity is such a weak force, why is it so dominant?

Because there's no 'anti gravity' and because the range of gravity is infinite. The weak force and the strong force are a lot stronger than gravity, but they are mediated in such a way that they don't reach very far. read more

The weak force and the strong force are a lot stronger than gravity, but they are mediated in such a way that they don’t reach very far. To them, most of the universe is opaque: they will react with something before they get very far (like light would react with the electrons in opaque material). read more

If you determine the coupling constants of all different forces, you discover that, in decreasing order, strong, eletromagnetic and weak forces are much, much stronger than gravity. You need around $10^{32}$ (that is 100,000,000,000,000,000,000,000,000,000,000) times more "stuff interacting" to get around the same energy scale with gravity if you compare it with the weak force. read more