I saw a movie "The Elegant Universe" by Brian Greene. It was said that the reason for gravitational force being weak as compared to the other forces is that it is confined mostly to higher dimensions and that we feel only the effects of it in the known dimensions. My question is, why should we not just accept that gravity is a weak force by nature. Why do we have to set out explaining that it is not a weak force ? Does String Theory say that gravity is indeed comparable in strength to the Nuclear and EM Forces ? Do we really need to prove that the forces are comparable in strength in order to obtain a unification theory? Please educate me if I have got this understanding wrong.