Perhaps the most amazing aspect of our Universe is that we only see about 1% (in the form of stars and bright nebulae) of its total mass and energy. A further 4% of its mass is in the form of dust and gas, but this leaves about 95% to be accounted for. A major element of this is, we believe, in the form of ‘dark matter’. It has been given this name as it does not interact with light and so is invisible but, due to the fact that it does have mass, it does exert a gravitational attraction on normal matter and this is how we have evidence for its existence.

The first question to ask is whether this invisible content is normal (baryonic) matter that just does not emit light, for instance gas, dust or objects such as brown dwarfs, neutron stars or black holes. These latter objects are called MACHOs (massive astronomical compact halo objects) as many would reside in the galactic halos that extend around galaxies.