Stephen Hawking was one of the greatest scientists and thinkers of our time, and is famous around the world because he wrote and talked about cosmology - the science of the universe - in a way that people could understand. He had a terrible disease called ALS, and for many years could only move and speak with the help of machines. But this did not stop him working to try and understand our world better. Will people travel in time one day? Where did the universe come from? Most of us cannot even begin to think about questions like this - but Stephen Hawking was always looking for the answers.