isn’t dark energy just a term for something we think must be there yet we can’t find it? go too many layers deep on speculation and it starts to feel meaningless and self-referential
I believe dark energy is the name for something that would be necessary to reconcile the model of an expanding universe with the amount of mass-energy that we can observe (even accounting for missing sources of gravitation, aka “dark matter”).
You would think that the gravitational attraction of all the matter in the universe would slow down its expansion, at least a little. However we observe that the rate of expansion of the universe seems to be accelerating over time, and it seems like something must be causing that. The cause is called dark energy, but you’re right, we don’t really know what it is.
It’s analogous to the situation with dark matter. Again we hypothesise some additional factor that causes behaviour we wouldn't otherwise expect. We attach the terms energy and matter to them, because those are the things that generally cause the kind of effects we observe, but they’re placeholder terms and it’s always possible the observations or our theories are incorrect or incomplete.