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Dark Matter

  Dark matter is a kind of matter hypothesized in astronomy
  and cosmology to account for gravitational effects that
  appear to be the result of invisible mass. Dark matter
  cannot be seen directly with telescopes; evidently it
  neither emits nor absorbs light or other electromagnetic
  radiation at any significant level. It is otherwise
  hypothesized 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

What is Dark Matter? A Mystery of the Universe


  Explanation: The matter in galaxy cluster 1E 0657-56, fondly
  known as the "bullet cluster", is shown in this composite
  image. A mere 3.4 billion light-years away, the bullet
  cluster's individual galaxies are seen in the optical image
  data, but their total mass adds up to far less than the mass
  of the cluster's two clouds of hot x-ray emitting gas shown
  in red. Representing even more mass than the optical
  galaxies and x-ray gas combined, the blue hues show the
  distribution of dark matter in the cluster. Otherwise
  invisible to telescopic views, the dark matter was mapped by
  observations of gravitational lensing of background
  galaxies. In a text book example of a shock front, the
  bullet-shaped cloud of gas at the right was distorted during
  the titanic collision between two galaxy clusters that
  created the larger bullet cluster itself. But the dark
  matter present has not interacted with the cluster gas
  except by gravity. The clear separation of dark matter and
  gas clouds is considered direct evidence that dark matter

Dark Matter Core Defies Explanation in Hubble Image


  MARCH 2, 2012: Astronomers observed what appeared to be a
  clump of dark matter left behind during a bizarre wreck
  between massive clusters of galaxies. The dark matter
  collected into a "dark core" containing far fewer galaxies
  than would be expected if the dark matter and galaxies hung
  together. Most of the galaxies apparently have sailed far
  away from the collision. This result could present a
  challenge to basic theories of dark matter, which predict
  that galaxies should be anchored to the invisible substance,
  even during the shock of a collision.

  The initial observations, made in 2007, were so unusual that
  astronomers shrugged them off as unreal, due to poor data.
  However, new results obtained in 2008 from NASA's Hubble
  Space Telescope confirm that dark matter and galaxies parted
  ways in the gigantic merging galaxy cluster called Abell
  520, located 2.4 billion light-years away. Now, astronomers
  are left with the challenge of trying to explain dark
  matter's seemingly oddball behavior in this cluster.

Dark Matter Core Defies Explanation in Hubble Image


Newton's law of universal gravitation

  Newton's law of universal gravitation states that any two
  bodies in the universe attract each other with a force that
  is directly proportional to the product of their masses and
  inversely proportional to the square of the distance between


Einstein's Law of Gravity

  General relativity, or the general theory of relativity, is
  the geometric theory of gravitation published by Albert
  Einstein in 1916 and the current description of gravitation
  in modern physics. General relativity generalizes special
  relativity and Newton's law of universal gravitation,
  providing a unified description of gravity as a geometric
  property of space and time, or spacetime. In particular, the
  curvature of spacetime is directly related to the energy and
  momentum of whatever matter and radiation are present. The
  relation is specified by the Einstein field equations, a
  system of partial differential equations.

  Some predictions of general relativity differ significantly
  from those of classical physics, especially concerning the
  passage of time, the geometry of space, the motion of bodies
  in free fall, and the propagation of light. Examples of such
  differences include gravitational time dilation,
  gravitational lensing, the gravitational redshift of light,
  and the gravitational time delay. The predictions of general
  relativity have been confirmed in all observations and
  experiments to date. Although general relativity is not the
  only relativistic theory of gravity, it is the simplest
  theory that is consistent with experimental data.

The NASA/ESA Hubble Space Telescope provides new evidence for 
dark matter around small galaxies

  Peering into the tumultuous heart of the nearby Perseus
  galaxy cluster, Hubble discovered a large population of
  small galaxies that have remained intact while larger
  galaxies around them are being ripped apart by the
  gravitational tug of other galaxies.

  The Hubble images provide further evidence that the
  undisturbed galaxies are enshrouded by a "cushion" of dark
  matter that protects them from their rough-and-tumble

Hubble Studies Dark Matter Filament in 3-D

Astronomy Picture of the Day Archive