Genuinely we can't tell what it is. We once thought it was just a normal pull due to mass until Einstein proved us wrong during a solar eclipse where we could see stars that shouldn't be visible from our current position in orbit. Then we get into how it works, WHICH THERE IS NO TELLING AS THERE ARE TO MANY GOD DAMNED VARIABLES INVOLVED.
Gravitons, like the rest of the standard model, are real. In order to measure them, we need incredibly high energy collisions.
In attempting to do this, our universe was created on accident. This is because our universe is in a black hole. But the cool shit is that black holes are really portals into our own universe. Instead of the energy spontaneously appearing as measurable particles, it's dark energy causing the expansion of the universe. The Dark energy ends up being gravitons, because gravitons are a boson and only interact with matter with the gravitational force. the black holes, then, take in "normal" fermions and bosons and spit out the gravitons. The graviton release causes an expansion of space time by "pulling" space time with it.
I respect physicists for doing something I can't. Realistically if I had the money and time, I could pursue a degree in it, I just need my work to be less abstract to stay sane.