A graviton is a hypothetical particle that carries the gravitational force. The particles themselves only exist at the most fundamental of levels. They are smoky, referencing the fact that particles do not have a defined location or momentum (Heisenberg's Uncertainty Principle).
They are found inside the Quantum Level .
A graviton, if it exists, would have a cross section many times larger than the Planck Length. A graviton would not exist on the Planck Level because the strings, which make up gravitons and all other particles, are on the same level.