Glad it helps. I don’t like any of the analogies – Ballon, Fruit Bread, Donuts are all shown with 3D boundaries. Big Bang analogy is thought to be an explosion. That’s why there are all these questions about center of the universe and boundary. There is a lot of discussion about the observable universe, which I understand because that is all we can observe. But it seems to me that the original question relates to the total universe. I suppose that the questions about an imagined center of the universe arise from the assumption that the high original density had a source. A source implies causality, so if time started with the BB, there was no cause and therefore no source. Can somebody help me with the question: if we are saying that the original high density occurred everywhere, where does all this mass go when the density falls? Is it assumed that dark energy has mass? With the observed expansion of the universe, which we assume also to be the case in the non-observable universe, are we saying that there is an increasing emptiness (not containing dark energy or anything else) which is reducing mass per cubic light year? Or does dark energy maintain mass density? If not, where does the emptiness come from?