What exactly are the evidences for the "Q source theory"?

When I say the "Q source theory" I basically mean any of the modern theories that deviate from view that the Gospels were written by the apostles etc.

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    This is very broad. Entire books are written on this subject. I'd recommend beginning by reading about this (even on Wikipedia) and then asking something more specific, such as whether scholars believe a certain pericope exists in the Q source materials, etc.
    – Dan
    Oct 27, 2020 at 15:39