What is gravity, actually?

Newton described gravity as a force of attraction between masses, proportional to their masses and falling off with the square of the distance, and that description remains accurate enough to fly spacecraft. Einstein's general relativity replaced it with something stranger and more complete: mass and energy curve spacetime, and what we experience as gravitational attraction is objects moving along the straightest possible paths through that curved geometry. Under this view the Earth does not pull you down so much as your natural path through curved spacetime is interrupted by the ground pushing up.

The evidence for the relativistic description is substantial and practical. It correctly predicted the precession of Mercury's orbit that Newtonian mechanics could not explain, the bending of starlight passing near the sun confirmed during a 1919 eclipse, and gravitational time dilation, which is a working correction built into GPS satellites. Gravitational waves, ripples in spacetime predicted a century earlier, were directly detected in 2015 by LIGO from the merger of two black holes.

Two open questions remain genuinely unresolved. General relativity and quantum mechanics have not been reconciled into one framework, and gravity is by far the weakest of the four fundamental forces, weak enough that a small magnet lifts a paperclip against the entire mass of the Earth. Why it is so weak is an active question in physics. And the observed motion of galaxies requires far more mass than we can see, which is the dark matter problem, with the alternative being that our description of gravity is incomplete at galactic scales.