Answer to Question 1
Studies of the gravitational interaction of clusters reveal they have 10-100 times more mass than is visible.
Answer to Question 2
The massive visible star is pulled around by an invisible massive companion in a 5.6 day orbit. If the visible star is a giant of 30 solar masses, then the companion must be close to 10 solar masses, well over the 3 solar mass limit for black holes. Also, the matter pulled off the visible star is seen to be flowing toward the dark companion, where it generates much X-rays in the accretion disk.