  We re-derive the possible dependence of the redshift with very high energy
(VHE) $\gamma$-ray photon index. The results suggest that the universe to VHE
$\gamma$-rays is becoming more transparent than usually expected. We introduce
the extragalactic background light (EBL) plus the photon to axion-like particle
(ALP) oscillations to explain this phenomenon. We concentrate our analysis on
70 blazars up to redshift $z \simeq 1$. Assuming this correlation is solely the
result of photon-photon absorption of VHE photons with the EBL, which finds the
deviations between the predictions and observations, especially at redshifts
$0.2 < z < 1$. We then discuss the implications of photon-ALP oscillations for
the VHE $\gamma$-ray spectra of blazars. A strong evidence shows that: 1) the
EBL attenuation results that the VHE $\gamma$-ray photon index increases
non-linearly at the ranges of redshift, $0.03 < z < 0.2$; 2) the photon-ALP
oscillation results in a attractive characteristic in the VHE $\gamma$-ray
photon index at the ranges of redshift, $0.2 < z < 1$. We suggest that both the
EBL absorption and photon-ALP oscillation can influence on the propagation of
VHE $\gamma$-rays from distant blazars.
