Horizon discovers that 96% of the Universe is missing and only 4% is made of stuff we understand. Some scientists claim they know what the missing matter is. Others insist that nothing's missing at all and that the real problem is far, far worse. They say that Newton was wrong about gravity and that science needs to be rewritten. Dark matter, dark energy and variable gravity are put under the spotlight, as the world's leading cosmologists attempt to explain the biggest problem in science today.