  We explore a new horizon of modified gravity from the viewpoint of the
particle physics. As a concrete example, we take the $F(R)$ gravity to raise a
question: can a scalar particle ("scalaron") derived from the $F(R)$ gravity be
a dark matter candidate? We place the limit on the parameter in a class of
$F(R)$ gravity model from the constraint on the scalaron as a dark matter. The
role of the screening mechanism and compatibility with the dark energy problem
are addressed.
