We know there is mass out there, but it's not emitting light. Remember that stars closest to the center orbit faster than stars further out. If the stars that formed in the arms stayed there, the spiral arms would wind up in a few billion years. This would mean that spiral arms are a short-lived phenomena, and that the arms would be all wound up now, but we see them in other galaxies, both younger and older ones. We see that spiral arms have most of the youngest, hottest, bluest stars in the Galaxy.
Spiral arms result from waves of star formation going through the disk of the galaxy.
More like sound waves, which compress and decompress the air as they travel. Wave goes by an area, everything in area affected by the increased density stars get packed together as the wave crest hits them. Most importantly, molecular clouds will also get compressed as the wave goes by. Clouds collide with each other, get larger and more dense, so star formation is more likely to happen in the clouds in the spiral arms. Lots of star formation will go on as the wave crest hits the molecular clouds.