Dark matter is a form of matter thought to account for approximately 85% of the matter in the universe and about a quarter of its total energy density.  Its presence is implied in a variety of astrophysical observations, including gravitational effects that cannot be explained by accepted theories of gravity unless more matter is present than can be seen. For this reason, most experts think that dark matter is abundant in the universe and that it has had a strong influence on its structure and evolution.

Credit: http://physicsworld.com/