By saying 'they are not very well known' - then you're including every human capable of listening to them - and trust me, they're not all music connoisseurs. If you exclude the ignorant masses, they're about as popular as it gets.
Oh, if you get all "OMG James Labrie is a God," don't bother with his solo album, it sucked. OSI and Liquid Tension Experiment are more or less side projects of Dream Theater, and they're both pretty good.
If you want some help exploring the genre, try Symphony X (albums: The Divine Wings of Tragedy, V: The New Mythology Suite, The Oddysey), Ayreon, Fates W