One idea from cosmology is that galaxies are all moving away from each other. Naively this could be taken to mean that they are expanding into something. Popularly this could be seen as them moving outwards into an area of completely empty space at the 'edge' of the universe.
However, if the universe is infinite and unbounded and uniformly populated, there is no empty volume for the galaxies to move into. Therefore logic dictates that the space between galaxies must be expanding.
Let us suppose this is true. Then
Is it also the case that the space within the galaxies is expanding.
What about the space in our solar system.
Where do we stop? Under this theory is the space inside an atom expanding?