Galaxies and stars are made up of stuff: particles that we can see and measure. These particles might be very small - a hydrogen atom is only 5 hundred-millionths of a millimetre across, for example - but we can understand them and what they are up to. However, only about 3% of the total mass in the Universe is made up of particles like this. The remainder is mysterious. We can't see it. But we know it's there, because it affects the movement of galaxies.

The speed at which galaxies move in clusters suggests that a cluster's total mass is much greater than stuff can account for. Observing single galaxies suggests a similar shortfall. The different speeds at the centre and the edge of a galaxy as it turns allow us to estimate its size and the distribution of matter within it. And it seems that there is more mass in galaxies than can be accounted for by adding up the mass of all the stars, gas and dust.
The extra mass has been named dark matter. Galaxies seem to be surrounded by halos of dark matter that are probably about 10 times as big as the visible part.

Dark matter makes up just under a quarter of all the mass in the Universe. But we don't know what it consists of. One theory is that it is made up of WIMPs - weakly interacting massive particles that are heavy but don't change the path of 'ordinary' particles they meet.

	This is an extract from 'Blast off!', chapter 1 of the Voyage in Space and
	Time booklet