  We study coherently oscillating massive gravitons in the ghost-free bigravity
theory. This coherent field can be interpreted as a condensate of the massive
gravitons. We first define the effective energy-momentum tensor of the coherent
massive gravitons in a curved spacetime. We then study the background dynamics
of the universe and the cosmic structure formation including the effects of the
coherent massive gravitons. We find that the condensate of the massive graviton
behaves as a dark matter component of the universe. From the geometrical point
of view the condensate is regarded as a spacetime anisotropy. Hence, in our
scenario, dark matter is originated from the tiny deformation of the spacetime.
We also discuss a production of the spacetime anisotropy and find that the
extragalactic magnetic field of a primordial origin can yield a sufficient
amount for dark matter.
