A wormhole that takes longer to traverse than ambient space. Useful for certain government functions and, of course, lawyers, but not much else. Maybe we could

A wormhole that takes longer to traverse than ambient space. Useful for certain government functions and, of course, lawyers, but not much else. Maybe we could put our garbage in it and then destabilize the entrances. Maybe some galactic civilization has been doing this for millions of years and it turns out that dark matter is really just old magazines and used condoms and coffee pods and such.