narrator: perlmutter and riess, together with riess' team leader brian schmidt, shared the 2011 nobel prize in physics. (applause) but their discovery had opened up a whole new mystery. what was fueling the acceleration? scientists called it dark energy. perlmutter: something like 70% of all the stuff in the universe has to be made of this dark energy for the universe to be accelerating the way it is. and so it's possibly the most important stuff in the universe, and we don't know anything about it. i don't really know what dark energy is. we don't have an explanation that would satisfy really anybody, especially a physicist. and so it's blowing everybody's minds again. physicists call this the most important problem in physics because we don't know where this dark energy's come from. we can't calculate where it's come from. our models don't work. narrator: but despite the mystery, the discovery helped finally resolve the question astronomers had dreamed of answering for centuries: how old is the universe? perlmutter: you need to know that history of when was it faster, when was it sl