The answer is 'everywhere and nowhere'. First of all, remember that our entire foundation is the cosmological principle, telling us that no point in the Universe is special. If there were a particular point where the 'Bang' happened, that would clearly be a special point and violate the cosmological principle. Rather, space and time themselves were created at the instant of the Big Bang (unlike a conventional explosion where the material flies through pre-existing space). If we take any point in the present Universe and trace back its history, it would start out at the explosion point, and in that sense the Big Bang happened everywhere in space. In another sense, the location of the Big Bang is nowhere, because space itself is evolving and expanding, and it has changed since the Big Bang took place. Imagine the Universe as an expanding sphere; at any instant 'space' is the surface of the sphere, which is becoming bigger with time (again I'm thinking of a two-dimensional analogy to our real three-dimensional space). The place where the 'Bang' happened is at the centre of the sphere, but that's no longer part of the space, the surface of the sphere, in which we live. In particular, being constrained to the surface of the sphere means we are unable to 'point' to the place where the explosion is supposed to have happened. However, all the points in our current space were once at the centre of the sphere, when the Big Bang took place. what do you say about this?