> long interest in cosmology

Good, let's start with that expression of interest and run with it.

Here's my high level challenge to let you reflect for yourself (privately) on how interested you really are.

> those three proposed mechanisms are very hairy, in Occam's Razor terms

Write down a standard gravitation-containing effective Lagrangian or the action on the one hand, and an alternative (see e.g. Famaey & McGaugh 2012 in Living Reviews in Relativity, 15, 10, §§6-7) and run it through your favourite character-counting tool.

Then try an analytical or numerical solution for a simple toy model in either of them.

Then tell us which one you think is hairy and why, and whether your idea about how to assess hairiness has differed during this little study programme.

There is local help available for you in doing this at a local community college or small state-funded university (or local equivalents) in the physics department. It won't take long to walk you through it at a STEM undergraduate level (I would start with the Einstein-Hilbert action with the cosmological constant and proceed to perturb that in small steps [1]), go deeper if you can follow the mathematics, and aim you at further resources if you are honestly interested in them).

I think this will be more satisfying for your intellectual curiosity than making self-admittedly uninformed negative comments on Internet forums (even this one).

I'm pretty sure that if you asked in the comments here you'd have several people offering to help put you in touch with someone local to wherever it is you are. Satisfying honest intellectual curiosity is catnip to academics.

> seriously-hard problems in cosmology that are not explained

Yes, you're right. Cosmologists collectively are working towards a consistent and complete and explanatory model, but are not done, despite the "corpses" of many many ruled-out possibilities. (It is easy to dream up a possibility, hard to write it down consistently, and almost always really easy to kill it on first contact with real astronomical data; Einstein went through this many times including on papers he sought to have published).

However, I think you can get closer to understanding what those problems are, and why they're hard, and what we know can't solve them, assuming you retain your interest.

> [DE] ... could in fact be explained by a single simple, theory.

The Cosmological Constant is literally just a scalar multiplier on the curvature. Practically the only way to make it simpler is to zero it out, which is exactly what had been done before the discovery of the apparent acceleration of the expansion of the universe. Dark energy research (on the theoretical side) is principally, "in order to improve the accuracy of our modelling compared to data from various observatories, do we need to change the Cosm.Const., which is the \Lambda in the \Lambda-CDM model, into a more complicated function? If we do so, what observables are generated (especially if they would appear in already-gathered data (e.g. DES)?". [See the parenthetical three paragraphs up...]

- --

[1] An actual lecture for grad students (and some undergraduates) at McGill University in Montréal has this section: https://www.youtube.com/watch?v=pjn_wZ-1ne0 (from https://www.mcgill.ca/study/2021-2022/courses/phys-514 ETA: n.b. at the 41min58sec mark re my approach above) Please feel free to reply in the comments about whether this is interesting but too difficult to follow, and I will try to aim you at something more suitable for you. (You also will probably find lots of HN commenters directing you at e.g. PBS Spacetime and similar, but I think that's not really a path to understanding what hairy even is). Really I'm guessing you'd be better off with an informal tutorial session, which might cost you a coffee and a decent sandwich brought to some physics teacher's office, and some diplomacy beforehand (don't start with "I want you to show me how to argue on the Internet about physical cosmology's failings", for instance, much less "give me a devastating 240-character slogan I can repeat until all the relevant physicists get different jobs or mute me").