The universe has been expanding ever since the Big Bang some 14 billion years ago. But in 1997 astronomers discovered to their shock that the universe is not just expanding, but that the rate of expansion is slowly accelerating. They made this discovery by carefully measuring special types of supernova from distant galaxies, which glow with a known brightness. It is not understood what is causing this expansion; therefore it has been christened 'dark energy'. It does however make up the majority of the mass-energy of the known universe.