In a recent collision of galaxies known as the Bullet Cluster, scientists think they have proof of dark matter. In this cluster there are galaxies and intermixed gasses. When there was a collision, the gasses slowed more than the galaxies. Measurements showed that large amounts of mass that that should have fallen behind with the gasses continued ahead with the galaxies.
"This provides the first direct proof that dark matter must exist and that it must make up the majority of the matter in the Universe." said study leader Doug Clowe, from the University of Arizona.
Observations of our universe have not "fit" into our theories about how the universe should behave. To solve this dilemma, scientists "invented" what they call dark matter and dark energy. Dark matter refers to matter that does not emit or reflect enough electromagnetic radiation (light) to be detected directly, but whose presence may be inferred from its gravitational effects on visible matter. An example is when light from a known star is bent too much as it goes near a galaxy. The explanation is that the galaxy must have some invisible (dark) matter. Several other observable phenomena support that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than is directly observable, indicating that the remainder is dark.
A second explanation is that gravity does not behave the same way in galaxy clusters light years in size as it does on Earth.
A proposed alternative to physical dark matter particles has been to suppose that the observed inconsistencies are due to an incomplete understanding of gravitation. To explain the observations, the gravitational force has to become stronger than the Newtonian approximation at great distances or in weak fields.
Source: NASA press release