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 } \lesssim 3 \times 10 ^ { -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 .