  We review extragalactic $\gamma$-ray propagation models with emphasis on the
electromagnetic (EM) cascade process in the magnetized expanding Universe. We
consider cascades initiated by primary protons of ultra-high energy accelerated
by blazars and show that the observable spectrum is similar to the universal
spectrum of a purely EM cascade. We also present a detailed calculation of the
observable angular distribution for the case of EM cascades developing from
relatively nearby (<20 Mpc) sources. Finally, we calculate the point-like
source differential sensitivity of a novel liquid Argon time projection chamber
$\gamma$-ray telescope and show that its sensitivity is several times better
than the Fermi LAT sensitivity in the 100 MeV -- 100 GeV energy range.
