Sidestream music is an alternate name for "alternative music
." Just as popular and formula-ridden as the mainstream
, but with a different sound.
Early on, alternitive music was an blanket phrase for a wide variety of music that was not easy to label. It was just new music that didn't fit into the standard radio format. Many (now recognized) genres of music had at one time been called alternative. Now, the idea of "alternative music" being different, is a joke. It is no longer a blanket phrase for many types of music, but a specific post-punk or punky sound that is just as popular as any "mainstream music." Terms like mallternative have been used that note this irony.