Near pristine gas in the early Universe

The first stars formed from a pristine material and are fundamental in our understanding of the Universe. Yet, observationally, we know nothing about them as we cannot see them. This thesis addresses this gap by looking for gas clouds that may contain the elements synthesized in the first stars during their short existence and dispersed by their explosive deaths. This “near pristine” gas can be studied using the absorption lines it leaves in the spectra of bright background quasars. In this work, we discovered three such “near pristine” systems and one seemingly pristine cloud, even 1.5 billion years after the Big Bang.