They are only talking about the surface of the balloon. For the purpose of the metaphor only the surface exists. Imagine infinite space. Now imagine we divide infinite space into a grid of 1 meter cubes. Now imagine we double the size of every cube. That’s more or less how expansion works, it is an expansion of space everywhere, not outwards like an explosion.
Doesn’t the balloon have a centre of mass that we can adjust coordinates to… all the time… at millions of km per second.
Okay, I see your point.
They are only talking about the surface of the balloon. For the purpose of the metaphor only the surface exists. Imagine infinite space. Now imagine we divide infinite space into a grid of 1 meter cubes. Now imagine we double the size of every cube. That’s more or less how expansion works, it is an expansion of space everywhere, not outwards like an explosion.
I’ve been struggling to not see it as explosion/outward expansion. This really helped!