Diffuse interstellar H atoms are the ultimate cosmic space mines and
represent a formidable obstacle to interstellar travel.
The paper[1] is an interesting read too, but of course there are other obstacles to relativistic spaceflight, such as reaching velocities capable of relativistic spaceflight.
[1] - http://www.scirp.org/journal/PaperDownload.aspx?paperID=2391...