On string axions and the dark universe

String axions have been proposed as candidates for solving a number of puzzles in cosmology. In this talk, I will focus on axions as dark matter and dark radiation. After a review on how string axions can occur in our universe, I will provide a string theoretical explanation of dark matter as composed by axions coming from type IIB string theory on Calabi-Yau orientifolds. Based on the latest bounds, I will show how likely it is for dark matter to be composed of such particles and in which abundance, and I will provide predictions on the preferred ranges of masses and decay constants. On the contrary, requiring the axions to lie in a particular range of the parameter space imposes constraints on the UV theory. I will focus both on the role of moduli stabilization and on the landscape of vacua from the Kreuzer-Skarke database of Calabi-Yaus. Finally, I will discuss axions production at the end of inflation and the implications for the proposed cosmic axion background.