The explanations I've heard for "dark energy" have been from folks not working in the field but it always comes across as "The universal expansion is accelerating, therefore - aliens".
My limited understanding is that dark matter and dark energy are placeholders, of sorts.
Dark "matter" means that it's something like matter, but not really. Something's wrong with the models, such that they don't match observations - or else our interpretations of the observations are somehow incorrect. So, dark matter is used. It's matter-like, but somehow it's quite difficult to detect.
Dark "energy" - same thing. It's kind of like energy, but not really. If that makes any sense. Something's making the Universe expand at an increasing rate. Energy though? Maybe, or maybe something different.
It seems that some of these fields like to do that. For example, my understanding is that electron "spin" doesn't refer to anything like the spin of a ball or disc. For example, you can have a particle with 1/2 spin, or spin 1/2, whichever it is. But I'm no particle physicist either, I just read a lot of things.
