We present an extragalactic background light (EBL) model based on a hierarchical galaxy formation model and make predictions for the gamma-ray attenuation up to z = 10. For the EBL model. We employ a semi-analytical galaxy formation model incorporating Population III stars. Although many aspects of EBL have been previously studied, a formulation together with Population III stars has been lacking.

Our model successfully reproduces various observations of cosmic star and galaxy formation history and is consistent with cosmic reionization data including the WMAP Thomson scattering optical depth constraints as well. Our model predicts the relatively transparent Universe to gamma-rays comparing to previous studies. We also find that the gamma-ray opacity of the Universe at z > 4 for gamma-ray photons below 60 GeV is still transparent. Future high-z gamma-ray observations by Fermi and CTA will provide a new window for understanding the cosmic early star formation history and cosmic reionization epoch.