Newton’s law had a flaw: It did not explain how one thing could act on another instantly, across any distance, with nothing in between. Nobody liked this “action at a distance,” including Newton.
Einstein’s theory of gravity contains a flaw, or maybe just a puzzle. Gravity doesn’t fit in with the universe’s other three fundamental forces: the electromagnetic, the weak, and the strong. The other three can all be described by quantum mechanics, which explains the three forces as fields created and carried by waves which are also particles. To date, gravitational waves remain undetected and gravitational particles called gravitons are probably undetectable. So at bottom this force that’s so familiar, whose quantification you read every day on your bathroom scales, is—what?
So what’s the matter with gravity? It may or may not be related to dark energy and it doesn’t fit in with the other forces. If it’s not a particle or a wave, then what else it might be is unclear. “And that’s where it sits now,” Turner says. “But wouldn’t you rather have no answer than the wrong answer?”