## Dark matter

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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 $\Omega _{DM} = \rho _{DM}/\rho _{crit}$ – 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]].