求助~麻烦牛人指点下这道逻辑题

笑脸多多1003

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.

Which of the following is an assumption underlying the passage above?

(C)the aggregate mass of visible stars in the galaxies mentioned above can be estimated with confidence.

这道题是白勇语法第37页的一道题，我看了以后怎么也想不通为什么选C。感觉上都没什么关系~
麻烦哪位牛人能指点一下。太感谢了！

笑脸多多1003

我好像发错了，发到阅读版上来了……