What is dark matter, and how is it affecting the universe?
Dark matter is the term used to describe missing mass in the Universe.
Observations of motions of large scale structures such as galaxies and clusters of galaxies can't be explained by the gravitational effects of the visible objects in the Universe.
Models predict that some 85% of the mass is invisible and only interacts with normal matter by gravity.
It is not know exactly what dark matter is. There is a theory that it consists of Weakly Interacting Massive Particles (WIMPS) which only interact via gravity and the weak force.
More recent theories suggest that dark matter can be explained as large numbers of black holes.
Other theories require rewriting aspects of general relativity.
So, dark matter is a hypothetical substance which explains the missing 85% of mass in the Universe.