Dark matter sounds like a pretty frightening concept. In reality though, it is like a sphinx without a secret – it acts very secretive and mysterious but it doesn’t appear to be hiding too much.
The concept of dark matter was created to explain a little quirk that came about from trying to weigh galaxies. There are two methods of doing this: Firstly, by measuring the rotation speed of the stars that orbit the centre of the galaxy, and secondly, a more wacky method of trying to count the stars. The first method calculates the strength of the gravitational pull on the orbiting star, which is related to the mass of the galaxy. The trouble with this is that weighing galaxies by looking at their gravitational pulls gives values which are 10 times higher than the values obtained by counting visible matter like stars and gas clouds. This means that 90% of a given galaxy is invisible to us.
Further, we can look at the way light from distant objects is warped by the galaxy when an object passes behind it, and guess at where in the galaxy the dark matter is – it turns out that the dark matter is distributed in a spherical halo around the flat spiral of the galaxy. I like to think of it as a huge ghost bubble that keeps everything together.
As for what dark matter is, there are many ideas. It could be really faint objects like rocks, or more exotic substances like new undiscovered particles called WIMPs. There is no universally accepted idea though. A recent survey by Christian Moni Bidin attempted to map nearby dark matter, but didn’t find any in the region around our solar system. If these results are confirmed, it could throw a spanner in the works of future attempts to understand dark matter.
Dark Energy, on the other hand, is an absolute fucking mystery to everybody. It arises because while the universe is expanding, the combined gravity of everything in the universe should be slowing the expansion, if only slightly. Everything we know about gravity suggests that it is an attractive force, and so there is no reason why the expansion of the universe should not slow slightly. Well, guess what, the rate of expansion of the universe is actually increasing! The mysterious energy that is driving this acceleration is called Dark Energy because nobody knows what it is. It’s as if you cut the engine on your car as you were driving up a hill, and then instead of slowing down the car sped up. We are at a loss to explain why.
The best idea is that empty space somehow carries an energy of its own; this energy is called the “Cosmological Constant”, and as the universe expands there is more space and therefore more of the mysterious energy. But nobody really knows why it is there or what it actually is.
As an interesting aside, Einstein included a cosmological constant in his original field equations as a way of ensuring the universe was in a steady state, neither expanding nor contracting. Later, when the expansion was discovered, he cursed himself for being so stupid.
Today, though, the cosmological constant is used to represent dark energy, and without it the models of the universe make no sense. This all occurred after Einstein’s death, but it turns out the ‘greatest mistake of his life’ is indispensible to modern cosmology, though certainly not in the way he had imagined.