They're not manipulating anything. They are observing that there are numerous objects which appear to have a lot more mass than is visible. Unless you think there is something wrong with our Classical view of gravity, then the obvious answer is that there is a lot more matter out there than we can directly observe.
Fucking hell, there's nothing worse than some self-appointed anonymous poster on the Internet who is some fucking arrogant and stupid that he thinks he understands something better than the scientists. And why is it that such arrogant fucktards always end up on