light dawns (on one)
Something suddenly becomes clear, certain, or fully understandable to one. The light dawned when I remembered that my grandfather had been stationed in Japan many years ago. It wasn't until I was able to say it out loud that the light dawned on me about what really happened.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
If light dawns, you suddenly realize or understand something. I didn't realize they were a couple till I saw them together last night and the light suddenly dawned. Note: You can also say that light dawns on someone. `Oh!' she said, as if the light had finally dawned on her. `I'm on the wrong floor, huh?'
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012