When a plane accelerates, we feel the back of the seat pushing us to keep up with the plane, but parachutists in free-fall feel no force at all as gravity accelerates them to the earth, so:
“It’s not the fall that kills you; it’s the sudden stop at the end.” Douglas Adams
Einstein concluded that there is no force so a free-fall equates to being at rest. He called his insight that the acceleration of gravity is the earth curving space and time around it “the happiest thought of my life!” Gravity then isn’t a force at all, but the earth changing space and time.
Gravity is indistinguishable from acceleration, so passengers in a rocket accelerating at 1g feel an effect like gravity. For example, they can sit down and have a cup of tea, just as they do on earth. Yet isn’t acceleration caused by particles? Not according to Einstein. He replaced Newton’s inexplicable force-at-a-distance by an equally inexplicable distortion of space and time. The stage within which objects act was then changeable not fixed, and gravity bends particles by changing it.
Einstein’s theory also explained Galileo’s finding that, but for friction, all masses fall at the same speed. A heavy object has more inertia, so it is harder to move, but if gravity is equally greater, the effects cancel. In a vacuum, a feather hits the ground at the same time as a ton of lead because gravity varies with mass as inertia does.
It was a brilliant solution, but it left the standard model with a force that its particles couldn’t effectively explain, as hypothetical gravitons contradict the general relativity claim that gravity acts by changing space and time, just as its idea that particles take fixed paths contradicts the quantum theory claim that they take all paths. Matter-based physics accepts the standard model because it sees no other option, but a quantum-based physics does because it lets quantum events exist.