The world around us is three-dimensional, but perhaps that's just an illusion. Some theories allow for extra dimensions of space, curled up or stretched in special ways. One such model depicts a five-dimensional universe with a "diluted" dimension (called a linear dilaton). In this picture, our familiar reality is merely a "surface" (brane) embedded in a deeper world.
In this construction, gravity behaves unusually. Ordinary gravitons, the carriers of gravity, acquire mass and can form a long-lived particle, a kind of isolated resonance. It interacts extremely weakly with ordinary matter, making it an ideal dark matter candidate. Its mass is less than 1 MeV (million electron-volts), thousands of times lighter than a proton. The particle arises through quantum corrections and accumulates in the universe gradually via the "freeze-out" mechanism.
But this is not the only candidate. The five-dimensional theory also predicts a continuous spectrum of excitations — something like a "holographic fluid." It also behaves like dark matter but is produced differently: its abundance depends on the reheating temperature after inflation. If the temperature was not high (less than TeV — tera-electron-volt), then both types of dark matter can coexist.
This approach ties cosmology and particle physics into a single story. Inflation sets the initial conditions, and extra dimensions provide natural dark matter candidates. Here, gravity is not just a force but a bridge to hidden sectors of reality.
Gravitons with masses below 1 MeV cannot decay into ordinary particles due to conservation laws, so they live longer than the age of the universe.
The idea of extra dimensions is explored in Terry Pratchett's novel "The Colour of Magic," but there it's elephants and a turtle, while here it's quantum gravity.