One of my driest, most boring, most methodical, and least-communicative astronomy professors (a cosmo-chemist) turned to the 8 of us in the Astro core curriculum one morning and said "Of course, keep in mind that all of this (gestures at formulae on the board) is relevant only so long as the Universal Gravitational Constant is truly constant; and we really don't have direct proof of that."
That'll snap you awake at 8:30am.
Bottom line is that I'm more prepared to believe that c is not constant over time than I am in something called "dark matter" or "dark energy".
And the thing that makes me smile is that once upon a time the concept of the cosmic ether made sense, because waves need something to propagate through. Then "the ether" became a stupid idea, because light waves are different.
And now we have to invent a magical something that no one can see and that is so pervasive throughout the cosmos that it actually determines the internal forms of the universe via gravity.
Hello, cosmic ether. Maybe we dumped you too soon. Will you take us back?