Start the major scale from each of its seven notes and you get seven modes, running bright to dark: Lydian, Ionian (major), Mixolydian, Dorian, Aeolian (natural minor), Phrygian, Locrian. Each lowered degree dims the light one notch — which is why Mixolydian's b7 sounds rock-relaxed, Aeolian's b3/b6/b7 sounds minor, and Phrygian's b2 sounds like a threat. Borrowed chords are just visits to the darker modes on the same tonic.