Clusters of galaxies or Galaxy clusters are joint collections of thousands of galaxies. It is assumed that these stars are hold in clusters by gravity. The sumed-up gravity of all these star objects in cluster is however regarded as inadequate to hold these stars together. That's why contemporary main stream scientists invoke the existence of mysterious dark matter to account for missing volume of gravity.
- Michail S. Turner. Dark Matter, Dark Energy and Inflation: The Big Mysteries of Cosmology 0h:37min:00sec/1h:11min:39sec. Arizona connection, Lectures series. Retrieved on 2012-10-14. “So how he did discover the Dark Matter? He was the first astronomer to study the clusters of galaxies. Clusters of galaxies are joint collections of 2000 or 3000 galaxies. F.Zwicky measured how fast these galaxies are moving. ...thousands of km per second ...He asks himself if these galaxies are moving so fast, what keeps the clusters together? And the answer is: Should be gravity, the gravity is a universal glue that holds things together. The planets in our solar system move fast, not quite this fast, and it is the gravity of our Sun that holds the Solar system together. There was little problem though: If you add up all the gravity of all the stars in all the galaxies it is not enough to hold the clusters together - it is off by factor of 50. Zwicky was bold enough to say that clusters must be held together by the gravity of unseen "Dark Matter".”