There are many more possibilities. Another is that we are late. The universe has already been bum rushed by von Neumann probes and other such things. This happ

There are many more possibilities. Another is that we are late. The universe has already been bum rushed by von Neumann probes and other such things. This happened a billion years ago. The cowboy era of the cosmos is now over. We are now in an era when mature highly advanced beings have some system of governance in place. This includes a kind of prime directive for simpler newcomers to protect them from premature contamination. We don't see these beings because our solar system is a nature preserve (due to our biosphere being in it), and their surveillance takes such advanced unobtrusive forms that it is undetectable to us. Radio silence in this scenario could be explained by distance, the use of cellular and complex modulations over inefficient broadcasting, or the discovery of something much better than radio that renders it obsolete. Perhaps intentional high amplitude signals toward naive worlds are illegal, as is detectable visitation. The only bugaboo with this speculation is where the megastructures are. Maybe highly advanced means so miniaturized and efficient that such big iron is pointless. The benevolent space brothers are immortal quantum computer brains at liquid helium temperature that consume a few watts of power, and space flight is done using low energy slow methods because when you are immortal who cares how long it takes. I'll call this one the wise old universe hypothesis.