When we look up at night, provided it is clear, we are met with a dark sky filled with a multitude of twinkling stars...
If we also happen to be scientifically inclined we might wonder at all those points of light - the superclusters, galaxy clusters, spiral galaxies, stars, quasars, etc. - and ponder the possibility that not all the mass that holds our Universe together is contained within this vision of luminosity...
We'd be right to ponder such things. The amount of luminous mass (mass that emits light) in a single galaxy, say, can be measured by how many points of light there are, and how intense each of those points are. This is not the only way however; we can also measure the amount of mass in the same galaxy by looking at the way it rotates - specifically its Galactic Rotation Curve.
The two do not add up. The employment of the second method infers a far geater mass than can be accounted for by luminous matter. When the discrepancy was first noticed (pre-1900), the extra matter was termed 'missing matter'. It was soon concluded however, that the matter was not missing, that it was there, but not visible. The new name was the now familiar 'Dark Matter'.
So what is this mysterious Dark Matter actually made of? Of what does it consist? The following link should whisk you away to some possible answers...
