Cosmic collision explained without dark matter
http://physicsworld.com/cws/article/news/31728

The Bullet Cluster -- a spectacular collision between two clusters of
galaxies -- is often cited as the best evidence to date for the
existence of dark matter. But according to John Moffat at the
University of Waterloo and Joel Brownstein at the Perimeter Institute
for Theoretical Physics in Ontario, Canada, the nature of the
collision can be explained using normal matter only -- that is, if
you\u2019re willing to accept their theory of modified gravity, or
"MOG" (Mon. Not. R. Astron. Soc. online early article).

Lying over three billion light years away, the Bullet Cluster
consists of a small subcluster ploughing through the centre of a
larger one at speeds of six million miles per hour. During such a
collision the stars pass each other by with little effect while the
intervening hot gas is slowed down.

To see how mass is distributed in the Bullet Cluster, scientists
examine how it behaves as a "gravitational lens" by bending the path
of passing light. In 2006, astrophysicists noted that X-ray light
emitted from very distant galaxies travelling through the Bullet
Cluster to Earth was bent more by the small galaxies at the sides
rather than the hot gas dominating the centre, which represents most
of the normal matter. This implied that there is an extra mass
component that is not associated with normal matter, leading many
physicists to hail it as strong evidence for dark matter. Assuming
conventional theories of gravity are true, dark matter is predicted
to comprise about 95% of all gravitating mass in the universe.

Moffat and Brownstein, however, argue MOG can provide a more natural
explanation by removing the need to invoke mysterious dark matter.
Essentially MOG adds extra terms to Einstein\u2019s theory of
gravitation -- general relativity -- that allow the gravitational
constant G to vary across space and time. The researchers modelled
the hot gas distribution using their MOG theory, and found the
gravitational force would be stronger as you move away from the
centre of the Bullet Cluster, explaining the stronger lensing seen at
the edges.

Moffat and Brownstein, however, argue MOG can provide a more natural
explanation by removing the need to invoke mysterious dark matter.
Essentially MOG adds extra terms to Einstein\u2019s theory of
gravitation -- general relativity -- that allow the gravitational
constant G to vary across space and time.