In 1998 two teams traced back the expansion of the universe over billions of years and discovered that it was accelerating. It was a startling discovery that suggests that more than 70 per cent of the cosmos is contained in a previously unknown form of matter, called Dark Energy. In this video, Brian Schmidt, leader of the High-Redshift Supernova Search Team, describes this discovery and explains how astronomers have used observations to trace our universe’s history back more than 13 billion years, leading them to ponder the ultimate fate of the cosmos. Brian’s work on the accelerating universe was awarded the 2011 Nobel Prize in Physics, jointly with Adam Riess and Saul Perlmutter.