Structure and shape of the Universe

If BIG-BANG theory is correct, then the shape of the expanding universe must be a hollow sphere with all the matter concentrated on the outer shell.But how come the structure of the universe is believed to be spacially homogeneous?

The concepts of Big - Bang and expanding universe are too wellknown. I do not understand why you are doubting my basic understanding of these concepts. Please quote the errors you indentified from my post.The shape of the universe I mentioned is my own idea steming out of the simple extrapolation of Big-Bang.All the administrators and members are most welcome to straighten me out.

Dear John,Thankyou very much for understanding and clearing my misunderstanding.

Dear John,Thank you very much for understanding (and clearing) my misunderstanding.

Dear Eta_carinae,Thanks for responding.My misunderstanding is cleared by Mr John.

Yes,the expanding balloon analogy is certainly misleading. Perhaps it can be compared with an expanding sponge, whose overall density decreases but maintains its spacial homogeneity.

Thank you for responding. I will get back after going through the references given by you.