Due to the big bang the universe is expanding... but you’d expect the expansion to be slowing down due to the drag of gravity. However, since the 1990s we’ve observed that the universe’s expansion is accelerating - the theory is that a force called “dark energy” is causing this acceleration. If the acceleration continues, there will be a point where distant galaxies will be accelerating away from each other at the speed of light. Since light is the universe’s “speed limit”, we would not be able to travel to these galaxies with known physics because even traveling at light-speed the galaxy we’re going to would be stretching away faster. Locally, however, due to gravity, it sounds like the paper is saying we would have regions of dense galaxies that stay closer together. Islands of matter in a sea of dark energy.