We know what 4% of the Universe is made of. But what about the rest?
There was a time, not so long ago, when science seemed to understand how the universe worked. Everything – us, the Earth, the stars and even exotic-sounding supernovae – was made of atoms which were all created at time-zero: the Big Bang. In between the atoms was nothing, a void: quite literally, 'space'.
But recently things have started to unravel. There is, it seems, a lot more to the universe than meets the eye. According to the best estimates, we only really know what about 4% of it is made of. But if only 4% is made of atoms, what about the rest? The rest is made of mysterious entities about which very little is understood, with equally mysterious names: dark matter and dark energy.
The accidental discovery
In 1974 the astronomer Vera Rubin, was working on a project investigating stars at the outer edges of galaxies. What she discovered was quite a surprise.
Shortly after the apple fell on his head, Newton famously declared that gravity was 'universal'. An apple falling on Earth obeys the same mathematical rules as an apple falling on the other side of the Universe. In the same way that the Sun controls the orbiting planets by exerting gravity on them, a spiral galaxy must be controlled by the gravity-giving black hole at its centre.
It has long been known that Pluto, at the edge of our solar system, travels much slower than Mercury, close to the Sun. In fact observations like these allowed Newton to pin down his laws in the 17th century. When Vera Rubin did her work on galaxies she expected to find that as you reach the edge of a galaxy the stars would be moving much slower than those close to the centre. But it didn't work out like that at all.
She found that almost all of the stars in spiral galaxies are racing around the centre at approximately the same speed. This was very strange. Could it be that Newton's laws weren't really universal and didn't apply in galaxies?
Questioning Newton seemed unthinkable, so the majority of scientists went down a different route altogether. Rather than variable gravity, they argued, there had to be something else in galaxies, something that was providing extra gravity. With extra gravity, the stars would be pulled harder, and would travel faster – as Rubin's observations suggested. And the name they gave to this extra stuff? Dark matter.
But what is dark matter?
Two men at Princeton University – Professors Peebles and Ostriker – looked further into dark matter. They even suggested that there was at least 10 times more of it than there was ordinary matter. But despite its growing acceptance, dark matter's real identity remained completely unknown. Nothing that particle physics came up with appeared to fit the bill. Even the newly-discovered neutrino had the wrong characteristics.
What was needed was something with mass but also something which does not interact with ordinary matter. Professor Tim Sumner from the Imperial College London believed he had the answer – a new, hypothetical particle called the neutralino. It is thought to have the right mass and exist in suitably vast quantities – but has never been detected.
If dark matter is everywhere in our galaxy, then it must be present here on Earth. In fact thousands of tonnes of the stuff must be passing through the Earth every day. It doesn't interact with ordinary matter, so it can pass straight through it, whatever 'it' is: us, the Earth, everything we're familiar with.
The bottom of a mine, away from the cosmic rays and atmospheric particles on the surface, is the perfect place to try to detect a signal. So that's exactly what Professor Sumner tried to do, with a detector located at the bottom of Europe's deepest mine on the coast in Cleveland, northern England.
If his team detected a neutralino, then a Nobel Prize would surely follow. But the search has so far proved fruitless.
Doubting Newton
Not everyone was so keen though. In 1974, while most scientists decided to pursue dark matter, Israeli astrophysicist Professor Milgrom tried something even more audacious – he tried to rewrite Newton's laws of gravity. Knowing this wouldn't exactly be welcomed by the rest of the community, he worked at his theory in private until he was ready to unleash it on the world in 1981.
He called it Modified Newtonian Dynamics (MOND) and used it to showed how gravity could be a little stronger than previously thought, across the huge distances that galaxies cover.
But surely Newton couldn't have been wrong? Milgrom continued to work on the theory and has since begun to attract admirers and recruit like-minded people. The longer the identity of dark matter remains a mystery, the more credence will be given to his ideas.
A deeper mystery
In 1997 Professor Saul Perlmutter opened another can of worms. While looking at the expansion of the universe, he accidentally discovered that not only were all stars and galaxies moving away from each other, they were doing so at greater and greater speeds.
This meant that our future selves might one day look up to a sky without stars (they'd all be too far away). It also meant that 'something' was pushing the stars apart. This anti-gravity force was completely new to science, but again what it actually was remained a mystery. It did however have a name: dark energy.
It turned out that the universe is 4% ordinary matter, 21% dark matter and 75% dark energy. That's a lot of stuff that no one really understands. Inevitably then, this Standard Model has its sceptics – not everyone believes that such a huge and important set of theories can be based on so little physical evidence.
Professor Mike Disney from Cardiff University even went as far as to suggest that this wasn't "physics at all – just fairies at the bottom of the garden".
In response the dark matter believers, led by Professor Carlos Frenk at Durham University, have produced impressive computer simulations of the Universe. These apparently show that dark matter and dark energy have been vital to the development of the Universe. Without their influence the galaxies, stars and planets, and indeed life itself, would never have come to be.
The results of the WMAP satellite survey appear to confirm the quantity of each of the 'dark' components. So despite the growing popularity of Milgrom's MOND idea in some quarters, dark matter still has the backing of the vast majority of scientists.
The standard model, with its officially approved mix of atoms, dark matter and dark energy, is the latest in a long line of brilliant ideas. Every civilisation since the year dot has had its own cosmological model. Every few decades or centuries, it has been replaced by something better.
Whether we are the privileged generation living in the time of the right idea remains to be seen. Is dark matter here to stay?
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