Abstract: I will explore the following topics: (1) I will discuss modeling the ultraviolet through infrared Extragalactic Background Light (EBL), which is the background light from all of the stars and dust that have existed in the observable universe. This radiation field can absorb gamma-rays to produce electron-positron pairs, and can thus be probed with gamma-ray observations, as well as optical and infrared surveys. I will discuss what we can learn about star formation and other physics from it. (2) I will discuss the brightest gamma-ray burst (GRB) ever observed, GRB 221009A. Fewer very-high energy (VHE) gamma-rays from this GRB are absorbed by the EBL than models predict, which may be an indication of new physics. I will discuss how VHE observations of this GRB can be explained by Lorentz invariance violation (LIV). Lorentz invariance is one of the fundamental symmetries of relativity, and its violation is consistent with some beyond Standard Model theories. (3) Astrophysical gamma-rays can also be absorbed by a much closer source of optical photons than the EBL, namely, by photons from the Sun. I will discuss how observations of absorption of astrophysical gamma-rays by solar photons could be used to probe LIV.