We report on the results of a search for \gamma -ray pair halos with a stacking analysis of low-redshift blazars using data from the Fermi Large Area Telescope .
For this analysis we used a number of a-priori selection criteria , including the spatial and spectral properties of the Fermi sources .
The angular distribution of \sim 1GeV photons around 24 stacked isolated high-synchrotron-peaked BL Lacs with redshift z < 0.5 shows an excess over that of point-like sources .
A statistical analysis yields a Bayes factor of \mathrm { log } _ { 10 } B _ { 10 } > 2 , providing evidence in favor of extended emission against the point-source hypothesis , consistent with expectations for pair halos produced in the IGMF with strength B _ { \mathrm { IGMF } } \sim 10 ^ { -17 } -10 ^ { -15 } \mathrm { G } .