BBC - Light and Dark 2of2 Dark (2013) RMAC

Episode 2 of 2
DURATION: 1 HOUR
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 at four billion kilometres from the sun there was precious little light to illuminate it and, like 89 per cent of all the atoms in the universe, it gives off almost no light.

In the middle of the 20th century scientists discovered something even stranger - dark matter - stuff that wasn't just unseen, it was fundamentally un-seeable. In fact, to explain how galaxies are held together and how they formed in the first place, there needed to be four times as much dark matter as there was normal atomic matter.

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.

Finally, Jim explores the quest to uncover the nature of dark energy and to see dark matter pull the first stars and galaxies together, a quest that involves peering into the darkest period in the cosmos's past.