Dark energy is just a placeholder for “we don’t know how this stuff works”. There is afaik no generally accepted theory about what it actually is. Whatever turns out to be the actual explanation is dark energy.
Yes, I think that’s why “doesn’t exist” is in quotation marks in the title.
I get what you are saying, and it seems reasonable if dark energy turns out to be something. If, however, dark energy turns out to be a placeholder for simply our misinterpretation of what is happening, it’s a little harder to say that dark energy exists.
The reason we say that is because we still see the effect of it, we just don’t see the cause. We see the universe expanding, but we cannot say why it happens. It’s not gravity because that obviously has the opposite effect and holds things closer together, so there must be some sort of force actively working against it. There’s a bunch of various hypothesis about it that could be the answer, but none of them can be proven, at least not yet. But the simple fact that we still see an effect by an unknown cause means that there still must be an unknown cause behind it, because things do not just happen without a reason.
