Dark matter is a type of matter hypothesised in astronomy and cosmology to account for effects that appear to be the result of mass where no such mass can be seen.
Dark matter cannot be seen directly with telescopes; evidently it neither emits nor absorbs light or other electromagnetic radiation at any significant level.
This three-dimensional map offers a first look at the web-like large-scale distribution of dark matter, an invisible form of matter that accounts for most of the Universe's mass.
image: NASA/ESA/Richard Massey, wikipedia
It is otherwise hypothesised to simply be matter that is not reactant to light
Instead, the existence and properties of dark matter are inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the universe.
According to the Planck mission team, and based on the standard model of cosmology, the total mass–energy of the known universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy.
Thus, dark matter is estimated to constitute 84.5% of the total matter in the universe, while dark energy plus dark matter constitute 95.1% of the total content of the universe.
Why it is needed
Astrophysicists hypothesised the existence of dark matter to account for discrepancies between the mass of large astronomical objects determined from their gravitational effects, and their mass as calculated from the observable matter (stars, gas, and dust) that they can be seen to contain.
The gravitational effects of dark matter suggest that their masses are much greater than the observable matter survey suggests.
Strong gravitational lensing as observed by the Hubble Space Telescope in Abell 1689 indicates the presence of dark matter—enlarge the image to see the lensing arcs. image: wikipedia/NASA, N. Benitez
Source adapted from: Dark matter. (2015, November 14). In Wikipedia, The Free Encyclopedia. Retrieved 08:19, November 15, 2015, from https://en.wikipedia.org/w/index.php?title=Dark_matter&oldid=690568206
Dark matter mights be like fine hairs crossing the universe
What is the composition or ‘cause’ of dark matter?
Astronomers and physicists currently have no firm ideas as to what it is made from. There are only a few kinds of hypothesis from things like weakly interacting but massive particles (WIMPS) to theoretical constructs related to string theory.
Further reading ( technical but accessible)
Dark Universe -Nature magazine