Dark Matter
"I do not know what I may appear to the world; but to myself I seem to have been only like a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me."
-Sir Isaac Newton, English mathematician and physicist (1642-1727)
What is Dark Matter?
The Composition of the Universe
Many scientists are conivinced that ordinary luminous matter constitutes only a tiny fraction of the Universe's total mass density. It is believed that between 70% to 90% of the matter in the Universe exists in undiscovered forms of dark matter. Still, dark energy's contribution to the total mass density is far greater than that of dark matter and ordinary matter combined.
Dark matter cannot be directly detected with optical equipment since it does not emit or reflect electromagnetic radiation. Nevertheless, its existence can be deduced by observing its gravitational effect on visible matter.
Evidence of Dark Matter
Visible matter, dark matter and light all interact with each other via gravity. We "see" visible matter because it can emit or reflect light. However, physicists believe that dark matter does not emit or reflect light and is, thus, invisible. Dark matter does interact gravitationally: for this reason, we know that dark matter exists because there is more gravity than the visible matter in the universe could produce.
Evidence of this "extra" gravity includes:
- The strange rotational velocities of stars in galactic disks can only be explained if much more mass is present in each galaxy.
- The relative bulk motions of galaxies combined with restrictions from nucleosynthesis show that the majority of the Universe's matter must be non-baryonic dark matter.
- The degree to which light is bent by galaxies and clusters indicates that 90% of the Matter in the Universe is missing.
- The density of the Universe is just right to make its geometry flat. Moreover, no viable theory of structure formation exists that does not contain a minimum amount of dark matter.
