Brane World: the dark plane stands for the 3 dimensional branes that we live in. It spans the large 3 spatial dimensions. The blue background is the bulk. It stands for the extra 6 spatial dimensions that are compactified to a very small size.

All standard model particles are open string modes. For example, the electrons, the photons, the quarks and the gluons are different excitations of the open strings. The ends of an open string must end on a brane. So the electrons and the quarks etc. cannot leave the branes. However, they can form structures like stars and galaxies inside the branes.

On the other hand, gravity is everywhere. In particular, the graviton is a closed string excitation mode. Since it has no ends, it can move freely in the brane or in the bulk.