Yes, Robin, I have the same problem. I cannot imagine infinite space, and I cannot imagine a limit to space.
But Forgiven has it slightly wrong. The BB is not an explosion in space, with an expanding border to the universe. This would be creationist Humphrey's theory, discussed in other forums. The BB view is expanding space with no limit, like the surface of an inflating balloon, with all points moving away from each other, and no border.
OK, so this involves imagining in ten dimensions or thereabouts. Just keep stretching your mind by imagining three impossible things every day before brewakfast.
Please don't ask whether the space in our bodies is also expanding, making us bigger (but maybe Joe Meert can answer that one - he usually has all the answers).
Nice reply Joz. I think that is the perfect answer.
But I also have this problem of imagining an expanding universe without picturing it expanding into empty space. I think we must stop thinking of it as an explosion or bang. Just think of an extremely small and dense universe getting bigger.
The balloon analogy 'assumes' tht space curves right around on itself, so that if you look far enough, you will see the back of your head (except that it wasn't there billions of years ago when the light left it - make sense of that one!).
Because the universe has no centre, there is no overall gravitational field, because stars and galaxies are pulling in every direction, all cancelling out. But there is still gravitational potential.
If the universe had a boundary, then there would be a gravitational field pulling towards the centre. At the centre all the pull in opposite directions would cancel, so there would be no field. The field would increase away from the centre, reaching a maximum at the boundary of the universe, and then falling away with the inverse square law beyond that (assuming a uniform density within the universe). If the universe were small enough (less that a million years old, or thereabouts) then the field would be strong enough to create an event horizon, and the universe could never have expanded beyond this horizon. There would just be a black hole. A big problem for Humphrey's theory. Within an event horizon time points inwards towards the centre. There is no other future.