Mysteries Between Stars

We know that solar systems form in a disk shape, with the star forming in the middle and any other rocks, presumably planets, form out from the center in whatever dust and gas remains. But what about the space between stars? Is it truly empty? And if there is something out there, how could we find it? How did it get there? For years, astronomers and chemists (believe it or not) have been trying to answer these questions and more.  The specific problem is that when we take a spectrum of a distant star, we see a collection of 400...