At one point in time before we had more knowledge of interstellar conditions there was hope that we could create advanced fusion rockets that would harvest hydrogen from the interstellar medium to use as fuel and reaction mass, allowing them to attain speeds of a pretty decent chunk of c. Now that we know that shit out between the stars is retardedly sparse,like less than an atom per cubic centimeter sparse, the idea is totally abandoned and for good reason.

But that wasn't always the case! The universe was much denser and less spread out billions of years ago,and it stands to reason that the interstellar medium was far more material rich. For a time, perhaps billions of years, a relatively easy form of interstellar travel was available that is now not available. Could earlier species have taken advantage of this? At what time did the ability to use interstellar ramjets fade away? could that have disrupted these civilations enough to explain the Fermi paradox?

Does anyone know about advanced modeling about the density of the interstellar medium going back over the universe's lifetime? We could figure out when these things could work and try to figure out if any hints of them being in use could be located.