From my understanding, space is not entirely empty. Infact, between the voids of space are molecules or maybe individual atoms of hydrogen just floating around. I understand that gathering massive quantities of this from space would be a difficult endeavor; however, if we could refine technology to use hydrogen thrusters and create a collection device that would collect the sporadic rogue hydrogen atoms floating in space to refuel our reserves could we travel space on an interstellar level indefinitely?
A second part to the question if for some reason the first is completely implausible. If the intake of hydrogen isn't sufficient enough to replenish the reserves we have, would it be possible to collect what we could from interstellar space and then fill up on hydrogen from a nearby comet, asteroid, star, or planet(oid) when we run low?
Very farsighted questions, I know, but a real question nonetheless.