Structure and shape of the Universe

The Big Bang was not an explosion. It was an expansion, and the expansion was an expansion of space itself, not the things within space. The Big Bang theory predicts that matter should be more or less evenly distributed through space (which appears to be roughly true on gigantic scales). It does not predict that "the shape of the expanding universe must be a hollow sphere with all the matter concentrated on the outer shell". That is merely your misunderstanding of the theory.See When the Big Bang exploded, why wasn't a large empty region created?, Big Bang, and Misconceptions about the Big Bang.