Dark matter is definitely one of the most sci-fi sounding names in current Physics and it has been puzzling astronomers for years. But what is all the fuss about?
Well, the story goes back nearly fifty years to a woman called Dr Vera Rubin who began studying the speeds of stars in galaxies. Most of the mass in a galaxy is at the centre so those stars closer to the centre should move fastest, and the speeds fall the further from the centre of the galaxy a star is. This is what can be predicted from Newton’s law of gravity and it is very well documented in other systems; Pluto moves a lot slower than Mercury for instance, because Pluto is a lot further from the Sun. But, to everyone’s surprise this was not at all what Rubin found; it appeared that all the stars in a galaxy orbited the centre at the same speed, regardless of their distance from the galactic centre.
This seemed impossible at first, as the stars toward the edge of the galaxies had speeds that were far too fast for the galaxies gravity to hold onto them. By rights, they should have been flung deep into intergalactic space. They were simply going too fast but were just happily orbiting, seemingly unware that they were in complete violation of Newton’s law of gravity. This meant one of two things; Newton’s age old, well tested and pretty darn accurate gravitational laws were wrong, or our understanding of galaxies was very wrong.
Unsurprisingly it seemed easier to update our views on galaxies than to tear down Newton’s law (admittedly Einstein kind of did this anyway but Newton remained good enough for all astronomer’s purposes). So, to ensure there was a reason why all the stars in a galaxy didn’t just fly off into deep space, there had to be some extra mass in the galaxy. This is exactly what Dr Vera Rubin postulated in the 1970s. Not just the sort of extra weight you put on over Christmas though, such was the speed of the outward stars that there had to be nearly 5 to 10 times as much extra mass in a galaxy as there was from the stars alone (which is a lot of pigs in blankets). It is this extra mass that was termed Dark Matter, a name first proposed by Fritz Zwicky in the 1930s. Observations proving the existence of this new kind of matter soon followed.
The name actually has pretty mundane origins really; Dark Matter is so called because it is 1. Matter and 2. Dark, in that we can’t see it. We know that Dark Matter is dark, i.e. cannot absorb or emit light, otherwise we’d have seen it ages ago and nobody would’ve needed to bother revolutionising our view of galaxies and indeed the universe.
We can map out the distribution of Dark Matter using Einstein’s gravitational lensing of light, whereby huge masses are capable of bending the light that passes nearby, which Dark Matter does plenty of. But what actually is this dark matter, because it can’t be any type of matter that we already know about because they all interact with light, which Dark Matter doesn’t. Well, nobody really knows, but a load of proper clever people are working to try and figure out exactly what is holding our galaxy, and all the others, together.
WIMPs are the strongest candidate currently for being dark matter. These Weakly Interacting Massive Particles tick all the boxes as to the properties that Dark Matter particles must have, i.e. being weakly interacting and massive. This proposed particle lies outside of the currently accepted standard model of fundamental particles. It does link up nicely with another theory, that of supersymmetry, which predicts the existence of particle with properties nearly identical to the WIMP. However, experiments at everyone’s favourite particle accelerator, the Large Hadron Collider, have not produced the results supersymmetry predicts they would. Further experiments directly aimed at observing the WIMPs themselves have also failed. These results put the theory of Dark Matter being WIMPs, at least in their simplest incarnation, into serious doubt.
There are many, many more possibilities for Dark Matter particles that are currently being explored with vigour across the world, all of which could turn out to be completely wrong of course. We know where Dark Matter is, we know what it does and we know how any particle should behave, but for now the question of what it actually is has the most exciting of answers: nobody knows. Yet.