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 a free-fall equates to being at rest, so there is no force. 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, who replaced Newton’s inexplicable force-at-a-distance with equally inexplicable distortions of space and time. He made the space-time fabric malleable, so gravity can move particles by changing it.
This 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 left the standard model with an effect that particles can’t explain. Its gravitons contradict general relativity, just as its particles taking fixed paths contradict quantum theory, so both relativity and quantum theory challenge the foundations of particle-based physics.