exactly what is mainstream music?

pretty much every modern rock band is considered 'mainstream' for
example MCR is my favourite band and I barely know any body who truly
knows them for their older songs only 'nana' and 'sing' but their
whole music style is much much harder than these songs and barely any
non MCR fans have actually listened to a real sample of their music.
the name MCR is well known but their music isn't does that make them
mainstream? also other bands that i like, like Rise against breaking
benjamin,papa roach, the blackout, bullets for my valintine ect...
i assumed that mainstream music is more like rhianna, lady gaga,
eminem and stuff like that but i have never heard any songs from the
bands i like on the radio what so ever (except nana) but yet people
like to look down on me because i'm too mainstream and they're
superior (in some way)
and i dont really understand how the music i listen to is mainstream
and how it's such a bad thing?
I'M FROM THE UK SO THE MUSIC MAY BE DIFFERENT (pointing out that i'm
not sure what mainstream is in the US)
i'm really just kind of sick of people who think they're superior
because they listen to music that people haven't heard of. (one of
them is actually a die hard foo fighters fan and i find that ironic)
A definition of mainstream music:
mainstream music
buy mainstream music mugs, tshirts and magnets
The worst music out there. It is usually for people that have low
IQ's, are conformist, and are afraid to explore other music out there.

It is always on the radio, Top 40, MTV, and everywhere you go! All of
it is crappy music created for people with no idea what good music is.