Indeed, by definition, the Universe comprises everything that exists. But then, how can it be expanding? And to what? To resolve this paradox, we must refuse at all costs to imagine that we are observing the Universe from a point outside ourselves: such a point does not exist, since everything is part of the Universe. In the same way, one cannot say that ‘Universe extends into a pre-existing space.
The Universe is expanding to itself, and this cosmic expansion must always be seen from an observer located within the Universe, and from this point of view, the expansion is very much like that of the dough that leavens. From this point of view, the expansion is very much like that of the rising dough.
As the dough rises, the distance between the grapes increases, as does the distance between the galaxies in the Universe. For the analogy to be perfect, of course, we must not look at the scene from the outside: we must dive into the dough with the grapes and forget that there is something outside the bread because the bread has become the Universe!
Another way of imagining things is to consider the membrane of a rubber balloon on which dots – galaxies – have been drawn. When the balloon inflates, the space between the dots increases, as does the space between the galaxies in the Universe. Again, one should not look at the balloon from the outside but imagine oneself as a two-dimensional creature trapped in the membrane. This membrane is the entire Universe, and the space inside and outside the balloon simply does not exist! If we do this, some of the questions we usually ask ourselves tend to resolve themselves, including the most troubling one: Where did the big bang take place?