http://www.newsweek.com/nasa-voyager-1-plasma-hum-interstellar-space-1590450

Voyager 1—the first man-made object to enter interstellar space—continues to detect plasma waves in deep space despite being far, far away from our sun.

Scientists think that interstellar space, which is the unimaginably vast near-vacuum in between stars, is filled with interstellar plasma.