Inflation and Dark Matter From String Theory

In this thesis, I have described my research work in particle phenomenology and cosmology, arising mainly from a class of string models called fibre inflation. The work is presented as a merge of models. It is divided into two parts. In the first part, inflation from string theory, we show the construction of explicit examples of fibre inflation models which are globally embedded in type IIB orientifolds with chiral matter on D7-branes and full closed string moduli stabilisation, which has never been built before. For the second part, dark matter from string theory, we present two independent models describing dark matter. One work shows how a single-field string inflationary model, which allows the generation of primordial black holes in the low mass region, can account for a significant fraction of the dark matter abundance, while in the second one we present how stringy axions can be used to described the 3.5 keV line observed in galaxy clusters.