BBC - Light and Dark (2013) Part 2 Dark

Professor Jim Al-Khalili shows how scientists have used light to reveal almost everything we know about the universe.

Part 2: Dark

Professor Jim Al-Khalili tells the story of how we went from thinking we were close to a complete understanding of the universe to realising we had seen almost none of it. Today, our best estimate is that more than 99 per cent of the cosmos is hidden in the dark, invisible to our telescopes and beyond our comprehension. The first hints that there might be more out there than meets the eye emerged from the gloom in 1846 with the discovery of the planet Neptune. It was hard to find, because there was precious little light to illuminate it. In the middle of the 20th century scientists discovered dark matter - stuff that wasn't just unseen, it was fundamentally un-seeable. In the late 1990s scientists trying to measure precisely how much dark matter there was in the universe discovered something even more elusive out there - dark energy, a mysterious new force driving the universe apart that is thought to make up a colossal 73 per cent of it.

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