Interstellar
space (note the lack of caps on the word "space") is also, simply, the space between stars/
solar systems. Where, exactly, a solar system ends is not well defined
*; does the
Oort cloud billions of miles from
our sun count? It is, after all, held in place by the same gravity that keeps us whirling round and round.
Here in the sunless voids can be found wandering clouds of freezing molecular hydrogen, errant space probes, cascades of radiation and vanishingly weak television programs from the 1950s, and possibly dark matter. Should you choose to colonize hypothetical planets around Alpha Centauri, you'll spend most of your journey out here.
Bring a book.
*
Turns out that it appears to be defined pretty accurately. czeano sez: "most people (err.. most astronomers) consider the edge of a solar system to be the heliopause, where the solar winds become overpowered by interstellar ones." My waxing semi-poetic on solar borders should not be used for school reports, kids.