We understand the gravity from acceleration, but still don't know just how two objects actually apply forces to each other through a vacuum at millions of miles distance. Think of our sun, 92 million miles distance, and it keeps us in orbit. Otherwise we would fly off into space and all die from cold.
https://en.wikipedia.org/wiki/Graviton
Gravitons are one possibility, but we can't yet figure out how. Yet one thing for certain, gravity always works, and it is nice to know that in the morning when you put your foot out of bed it will go down, and not up to the ceiling 100% of the time.