Several tracers of gravitational field in astrophysical objects , such as
demonstrate that the dynamics of
galaxies and
galaxy clusters cannot be explained by the Newtonian potential produced by visible matter only. Moreover, cosmological data show that the
large scale structure in the Universe started to develop much be decoupling of photons at recombination and, therefore, much before ordinary matter could start clustering.
The simplest resolution assumes that a significant fraction of matter in the Universe does not interact with electromagnetic radiation. Current cosmological data determines this fraction to be
[1]
where
– the
critical density of the Universe.
This Dark Matter is then
universally distributed in all objects from
dwarf galaxies to galaxy clusters, and plays a dominant role in their dynamics. This elegant hypothesis poses, however, a major challenge to the highly successful
Standard Model of particle physics, as it was realized that dark matter cannot be made of known elementary particles.

References:- [1]^ D. Larson et al., ``Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Power Spectra and WMAP-Derived Parameters,'' Astrophys. J. Suppl. 192 (2011) 16 [arXiv:1001.4635 [astro-ph.CO]].