Right, but isn't that more a description of the situation than an explanation for the effect? "Oh, look, there's a repulsive effect adding up over large distances that is unnoticeable at (cosmic scale) small distances." And since that's work, there has to be an energy responsible because the potential to do work is more or less the definition of energy in physics. Last I heard, it was pretty well agreed that there's more to find out there ... of course, with science, we always find out there's more to find out there.
And again, I get that ... I guess what I'm saying is that it's my understanding that physicists don't think they have enough info to unify description and explanation into one thing on this issue. I'm one of those guys who reads a lot of lay science books and wishes news media did a better job of reporting on science both in terms of quality and quantity, but a long way from being any kind of expert. But when the experts are saying something needs better explanation, I tend to think it must.
And you'd be entirely correct in that. No argument there.
This is actually one of the things I've done some (research) work on. It's not entirely true that experts can't "unify description and explanation" on dark energy. It's I think more that there are some different possible explanations. Let me explain the situation, and then you can decide if I'm splitting hairs.
Within the context of general relativity, expanding space does not necessarily require work or reduce the total amount of energy. For example, normal matter scattered through space causes space to expand (in a way that slows down as the expansion of space spreads the matter out). In this case, if you were to define a region bounded by some set of galaxies, you'd find that the total energy in that region stays the same over time but gets less dense because that region has more volume.
What we see in our universe is that it is expanding and that expansion is actually speeding up. What that requires is that our universe be filled with something that does not dilute as fast as normal matter. The simplest possibility is that it is a constant energy density. What this means is that, as space expands, each cubic centimeter always has the same energy in it, so the "total energy" always increases! (I put that in quotes, though, because the "total energy" of a possibly infinite universe doesn't make a ton of sense in this context.) In any case, that's the usual cosmological constant that Einstein first thought about. But it's also possible that you have something like a cosmological constant that decreases slowly with time. There are lots of models you can make that will do that. I'd say that some make decent physical sense and a few just seem to be made up.
The other logical alternative is that Einstein's gravity is incorrect, so there is another theory of gravity. In this case, the universe likes to expand faster and faster even with normal matter (or maybe even nothing) in it.
I guess to be fair is that there's one other possibility, too. That possibility is that what we think we see in the measurements isn't correct and that the universe is actually slowing down. So you have to give an argument about how that might happen.
So it's not like there aren't explanations beyond description. The problem is more like having too many, and we will need more data to distinguish among them, so right now people just call the right one "dark energy." I think the "mystery" of dark energy gets played up a bit much to the public because people are quite passionate about this. A lot of it goes back to the cosmological constant. From a particle physics point of view, it must have a very tiny value (if it is the dark energy, it is 10^-123 times the naive particle physics value). For a very long time, people believed it must be zero and that the universe's expansion was slowing down. Now that we think the expansion is speeding up, a lot of people still want the cosmological constant to be zero.
So there are plenty of explanations. Of course, next we want to ask why does our favorite explanation come out the way it does. For example, if you like the cosmological constant, why does it have such a small but nonzero value? That's a bit trickier, worth a whole lot more than one post, and frankly controversial in a number of ways.
