Graviton

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.

Where Found
They are found inside the Quantum Level.

Other Info
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.