I learned that space is always expanding. Even between sun and earth, it's just gravity is stronger.
Imagine space as graph paper and there is a square object that fills it. Space expands, 1x1 cell becomes 2x.
The space expanded not the objects in it. The size of an object remains the same.
So if N amount of matter was present, it's still N. Just that before it was occupying 1 cell now 1/2 cell.
Did the density of object increase? something changed definitely. If it wasn't density then what changed.