[raattgift]

It is in fact the metric expansion of space. The spatial part of the Robertson-Walker spacetime metric expands equally along the time axes of a family of freely-falling future-directed worldlines (we can call them "Eulerian observers", and individual clusters of galaxies are good approximations).

That is, in the past, galaxy clusters are relatively close together, and in the future they are relatively very far apart.

If you need an image, think of a vase of cut flowers, with the stems tightly bound together at the bottom of the vase, and the flowers loosely separated at the top. Time is in the direction away from the vase's bottom. A super thin slice through a stem represents a snapshot of a galaxy cluster at a particular time in its existence.

https://s3-eu-west-1.amazonaws.com/images.linnlive.com/de6e1...

<https://media.istockphoto.com/id/578833902/vector/expansion-...> : time increases from the left to the right.