[denton-scratch]

/me not a cosmologist.

I think the story is that dark energy is indeed created, in the new emptiness resulting from the expansion of space.

<mumble> I believe it's supposed to be spacetime that expands, not 'space'. But it's beyond me to explain what that even means; as I understand it, spacetime refers to the whole Universe, across all of time. To 'expand' means 'to become larger over time'. But if the thing that's expanding includes time itself, then I'm bewildered.

[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.