Re: Relativity - Great! But what about String Theory, Dark Matter and Dark Energy?
Not quite.
Plus you have mixed up Dark Matter and Dark Energy.
Dark Matter explains the behaviour (gravitational) and morphology of the universe and things in it, ie. Galaxies and Galaxy Clusters.
It is hypothesised because it's the simplest explanation that is consistent with other observations. It's not really a circular argument because the stuff that's hypothesised is "matter that we can't see but which interacts only or mainly via Gravity". Nothing wrong with that. We don't know much else about it but lots of work is ongoing to narrow down the options.
More complex hypotheses may be possible too but usually it works to go with the simplest one that is consistent.
Dark Energy on the other hand is much more like the sky pixies you mention. It's a placeholder for an explanation for why the expansion of the universe appears to be increasing.
If and when someone comes up with a better explanation that is consistent with other observations and theory, then great, it will be replaced or explained in more detail. That doesn't mean we shouldn't have such a word.
Newtonian Gravity was much the same. What is Gravity? A force that makes things attract each other. Why do things attract each other? Because of Gravity.
Biting the hand that feeds IT
