If intelligent civilizations are common in the universe, where are they? Even assuming technology only a little better than Earth's, it would take less than 100 million years to populate the galaxy. Why haven't they come to visit? There have been a lot of ideas over the years. Last night I thought of yet another solution, and maybe an idea for a SF galactic civilization that doesn't require a warp drive or worm holes.
Suppose that our current understanding of physics is basically correct, and there isn't some exotic trick that allows us to exceed the speed of light. And suppose you want to design a galactic society, where it's practical for humans to range over the Milky Way and make occasional visits to Andromeda and other nearby galaxies, all within reasonable durations of subjective time. How do you do it?
First, it isn't impossible to travel galactic distances even in an unextended human lifetime. You just have to accelerate to relativistic speeds. This idea is a staple of science fiction; a great example is Joe Haldeman's The Forever War. The problem with traveling at relativistic speeds is the famous twin paradox: the traveler is fine, but when she returns home, she finds that her friends are all long dead, or in extreme cases, that the Sun has gone nova.
However, there's a way to avoid the twin paradox: have everyone travel at relativistic speed so they stay synchronized. Here's how it works. Once every 10 years (subjective time) the entire human race, which is spread all over the Milky Way, climbs into space ships, which take off and accelerate at 2 g long enough to reach nearly the speed of light (roughly a year). Then they all decelerate to their destinations, hang out for a year, and do it over again. Since they're all doing it simultaneously, they stay synchronized. They spend approximately equal subjective time traveling and staying put. Assuming that we manage to extend human lifetime to (say) a few thousand years, humans could range over the entire galaxy without breaking any physical laws or losing track of their friends and relatives.
Take this a step further. After some time, humans encounter evidence of another, larger and more advanced, galactic civilization doing the same thing. They figure out from available evidence the next synchronization point of the advanced civilization, and arrange to synchronize with it. They find that they are just the latest of thousands of civilizations to join. Possibly they find that there is an ongoing debate as to whether to join an even more far-ranging civilization that synchronizes even more infrequently and spans multiple galactic clusters. Imagine thousands of individual civilizations stuttering into the future, touching down only every few thousand years and leaving scarcely a trace,. Occasionally, tributary cultures join larger streams, populating ever more vast spans of time, but also populating time ever more sparsely.
The effect of all this is that the universe is densely populated with a huge, galaxy-spanning civilizations --- but only for brief, rare intervals. And all of human civilization has fallen into a gap in their timeline.