DARK MATTER

One of the biggest conundrums in modern astronomy is the fact that over 90% of the Universe is invisible. This mysterious missing stuff is known as 'dark matter'.

The problem started when astronomers tried to weigh galaxies. There are two methods of doing this. Firstly, we can tell how much a galaxy weighs just by looking at how bright it is and then converting this into mass.

The second way is to look at the way stars move. Everything in the Universe rotates. The Earth spins on its axis. The whole planet orbits around our parent star, the Sun. The Sun rotates around the centre of the Milky Way, along with the billions of other stars in the Galaxy, forming a huge cosmic dance. This rotation provides another way of weighing a galaxy. Studying how fast stars at the very edge move reveals the mass of the whole galaxy. The faster the Galaxy rotates, the more mass there is inside it.

But when astronomers such as Jan Oort and Fritz Zwicky did the two sets of sums in the early 1930s they hit a big problem. For every galaxy they studied the two answers didn't match. They were very confident that both methods were sound as they'd been tried and tested for many years. So they came to a startling conclusion - there must be stuff out there that we just can't see - and so they called it 'dark matter'. This dark matter was really important, as if it wasn't there then galaxies would fly apart as they spun round.

This might seem like a strange conclusion, but it's not really that bizarre. Imagine looking at a tower block at night. Although you can only see lights coming from some of the rooms, that doesn't mean that there aren't any more rooms in the tower. Just like these unlit rooms, dark matter can't be seen, because it doesn't shine.

Astronomers are currently hunting for this missing matter. It may consist of lots of strange sounding things like MACHOs, WIMPs and neutrinos. Or there may be new solutions involving dark energy or superstring theory. But whatever it is, finding it will help to answer one of the most fundamental questions in astronomy - what is the fate of the Universe?