we observe gravitational effects but we don't understand the mechanics of gravity.
"In theories of quantum gravity, the graviton is the hypothetical quantum of gravity, an elementary particle that mediates the force of gravitational interaction."
en.wikipedia.org
you can also model the universe without the use of gravity as a force by using curved spacetime instead.
partial readings of this (even though this does not mean that corresponding gravity-effects are not observed) can lead some to conclude that gravity does not exist.
NOTE: i didn't study physics at university and i didn't attend GT. i did some physics at school, read some books and watch science channels on youtube.
additionally gravity is much weaker than the other forces leading to some theories that gravity can cross (bleed into) universes / dimensions.
kip thorne is a theoretical physicist from the aforementioned caltech.
kip thorne worked as an advisor on the film interstellar.
this suggests why gravity was used in interstellar to communicate from higher dimensions.
(the sand, the drone, the watch)