The Fermi paradox is the discrepancy between the strong likelihood of alien intelligent life emerging (under a wide variety of assumptions), and the absence of any visible evidence for such emergence.
In this paper, the writers extend the Fermi paradox to not only life in this galaxy, but to other galaxies as well. They do this by demonstrating that traveling between galaxies – indeed even launching a colonization project for the entire reachable universe – is a relatively simple task for a star-spanning civilization, requiring modest amounts of energy and resources.
They start by demonstrating that humanity itself could likely accomplish such a colonization project in the foreseeable future, should we want to, and then demonstrate that there are millions of galaxies that could have reached us by now, using similar methods. This results in a considerable sharpening of the Fermi paradox.