  Several models of dark matter suggest the existence of hidden sectors
consisting of SU(3)_C x SU(2)_L x U(1)_Y singlet fields. The interaction
between the ordinary and hidden sectors could be transmitted by new Abelian
U'(1) gauge bosons A' (dark or hidden photons) mixing with ordinary photons. If
such A's have masses below the \pi^0 meson mass, they would be produced through
\gamma - A' mixing in the \pi^0 ->2\gamma - decays and be observed via their
decays A' -> e+e- in a detector. Using bounds from the SINDRUM experiment at
the Paul Scherrer Institute that searched for an excess of e+e- pairs in \pi^-
p interactions at rest, the area excluding the \gamma - A' mixing \epsilon >
10^{-3} for the A' mass region 25 < M_{A'} < 120 MeV is derived.
