As far as I understand $\ce{SO4^2-}$ structure, sulfur makes two coordinate bonds with two oxygen atoms and two double bonds are formed with oxygen after sulfur expands it's valance shell to 6. At this point calculating formal charge of those two coordinate bonded oxygen gives total formal charge of −2 and the formal charge on $\ce S$ and the double bonded $\ce O$ is zero. Thus the minus 2 charge on $\ce{SO4^2-}$ can be shown but initially all the atoms were neutral. While making bonds electrons were shared only.
How did excess 2 negative charge pop up?
Please help, I am certainly making some mistakes