Description: Dark matter is a proposed form of matter that has mass, but is completely transparent and does not emit light. It has been postulated as a joint explanation for a variety of phenomena related to gravitational interactions.

Early evidence for the existence of dark matter came from galaxies in galaxy clusters, which were found to be moving at comparatively high speeds. Postulating additional mass served to explain why, in spite of their speeds, the galaxies in question were gravitationally bound to their cluster instead of escaping. Measurements by Vera Rubin and others of the speeds at which stars and gas orbit in disk galaxies led to a wider acceptance of the concept of dark matter: there, the usual laws of gravity require considerable mass in addition to the visible matter in order to explain the observed high rotation speeds. More recently, gravitational lensing observations have indicated considerable non-luminous mass in galaxy clusters.

In cosmology, the expansion history of the Universe points to there being more matter in the Universe than is accounted for in the form of luminous matter. The common explanation for the growth of structure in the early Universe also relies on the presence of dark matter. For these reasons, the standard cosmological models are referred to as "Lambda CDM models", where CDM stands for cold (slow-moving) dark matter.

The nature of dark matter is a topic of intense research and debate both in cosmology and particle physics. Evidence for dark matter remains indirect, relying on observations of the effects of dark matter's mass on luminous matter or light. Several proposals exist for species of as yet undetected elementary particles that could make up dark matter, but the experiments set up to find direct evidence for such candidate particles have not so far been successful. There are also alternative proposed explanations that claim to account for the relevant observations without the involvement of new particle species.

Related Terms:

• Cosmology
• Dark Energy
• Galaxy
• Galaxy Cluster
• Gravitational Lens

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

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