Introduction

Recent progress in observational technics resulted in detection of magnetic field widespread well accross well beyond the galaxies into the galaxy clusters. It is thought that those fields are amplified from some primordial seed field via the dynamo mechanism. This seed field, hereafter refered to as Extralactic magnetic field or EGMF, if exists, may still stay in its initial state in the space outside the galaxies and galaxy clusters.

Detection technics

Various technics, such as Zeeman spliting and Faraday rotation, were applied to detect EGMF. Short overview of those technics is given in [5]. Very recently the observations of the suppresion of GeV fluxes of known TeV blazars were used to constrain the presence of EGMF ([1], [2], [3], [4])

Observational constrains

Usage of Zeeman splitting and Faraday rotation yielded to a constraint on the EGMF strength at the level of $10^{-9}$ G for the large correlation lengthes. Recently the lower limit on the stregth of EGMF was derived from the observations of the suppresion of GeV fluxes from TeV blazars. This lower limit in the most conservative approach was found to be at the level of $10^{-17}$ G for the large correlation lengthes of EGMF ([2]).