  Massive gravity with second and fourth derivatives is shown to give both
attractive and repulsive gravities. In contrast to the attractive gravity
correlated with the energy-momentum tensor, the repulsive gravity is related to
a fixed mass $m_x$, which equals a spin-dependent factor $f_\sigma$ times the
graviton mass. Therefore, particles with energy below $m_x$ are both dark
matter and dark energy: Their overall gravity is attractive with normal matter
but repulsive among themselves. Detailed analyses reveal that this unified dark
scenario can properly account for the observed dark matter/energy phenomena:
galaxy rotation curves, transition from early cosmic deceleration to recent
acceleration; and naturally overcome other dark scenarios' difficulties: the
substructure and cuspy core problems, the difference of dark halo distributions
in galaxies and clusters, and the cosmic coincidence. Very interestingly, Dirac
particles have $f_\sigma=1/\sqrt 2$, all bosonic matter particles have
$f_\sigma=0$, and the only exceptional boson is the graviton itself, which may
have $f_\sigma>1$.
