请教一道题

theonlyone

It has been hypothesized that much of the matter in the universe is “dark”, i.e. , unseen. Studies have shown that galaxies in many galaxy clusters are moving faster with respect to one another than they would if visible stars constituted all their mass. The studies suggest that the galaxies are moving under the gravitational influence of unseen mass in considerable quantity.

Assumption?

The aggregate mass of visible stars in the galaxies mentioned above can be estimated with some confidence.

Can anybody help to analyse it for me? Thanks.