Though to be fair, our expectations on what dark energy will do aren’t very well grounded.
We already have inflation and dark energy but no good reason to expect either of them to be there at all. Like, the book on dark energy I’ve seen go: ”here are a bunch of pretty good guesses”, thats not a field full of confidence in its expectations (though admittedly the book is 9 years old by now).
> We already have inflation and dark energy but no good reason to expect either of them to be there at all.
I have seen plenty of heuristic arguments for why we should expect inflation to be there, but I'm not really qualified to judge. None of them seem to have convinced enough physicists to be considered standard.
For the cosmological constant, I think it depends on what you consider the "natural" condition to be. The simplest way to derive the Einstein Field Equation is to start from the Einstein-Hilbert Lagrangian and vary it with respect to the metric. The justification for using the Einstein-Hilbert Lagrangian is that it contains the only scalar which contains up to second derivatives of the metric and is quadratic in the metric and its derivative, namely, the Ricci scalar. But that statement is not actually true: there's another obvious scalar that meets this requirement, namely a constant. So the most "natural" version of the Einstein Field Equation contains the cosmological constant: it should be there on theoretical grounds, and that means "dark energy" should be there as well.
Of course that also raises another problem, which is indeed one we don't have a good answer to at present: why is the cosmological constant so small?
We already have inflation and dark energy but no good reason to expect either of them to be there at all. Like, the book on dark energy I’ve seen go: ”here are a bunch of pretty good guesses”, thats not a field full of confidence in its expectations (though admittedly the book is 9 years old by now).