  Existing theoretical and observational constraints on the abundance of
magnetic monopoles are limited. Here we demonstrate that an ensemble of
monopoles forms a plasma whose properties are well determined and whose
collective effects place new tight constraints on the cosmological abundance of
monopoles. In particular, the existence of micro-Gauss magnetic fields in
galaxy clusters and radio relics implies that the scales of these structures
are below the Debye screening length, thus setting an upper limit on the
cosmological density parameter of monopoles, $\Omega_M\lesssim3\times10^{-4}$,
which precludes them from being the dark matter. Future detection of Gpc-scale
coherent magnetic fields could improve this limit by a few orders of magnitude.
In addition, we predict the existence of magnetic Langmuir waves and turbulence
which may appear on the sky as "zebra patterns" of an alternating magnetic
field with ${\bf k\cdot B}\not=0$. We also show that magnetic monopole Langmuir
turbulence excited near the accretion shock of galaxy clusters may be an
efficient mechanism for generating the observed intracluster magnetic fields.
