Q&A: Galaxies, Galaxy Clusters, AGN, and Quasars
We can't be completely sure that dark matter is pulling the core of the Fornax cluster and the outlying group of galaxies toward a common center of gravity. The directions of their motion could be a coincidence, but strong circumstantial evidence indicates that an unseen source of gravity is pulling them.
This evidence consists of measurements of the motions of many galaxies in the cluster with optical telescopes, and measurements with X-ray telescopes of the pressure of the hot gas that fills the cluster, which give the total mass of the cluster. The total mass determination shows that dark matter must comprise about 80-90% of the mass of the Fornax cluster.