Astronauts see light flashes when they are in space. It's radiation affecting the optic nerves.
Recipe for Undeniable Truth:
Be careful when you cut the fact, so you only get the most useful part of it and nothing more. If you get it wrong, you screw up the enire truth. Then you pull a crazy hypothesis out your ass, call it a theory, hold on to it, find litterature to support it (you only need the parts fitting your hypothesis). Don't worry if you it's hard to understand, that's what imagination is for. Then you belive it like you life depends on it. Now share it on the internet so more people can enjoy your Undeniable Truth.
Sceptics will try to destroy your reseach with their so-called facts, but they lie. Deny them, get emotional, ridicule them, give the target audience a good show. You are absolutly certian, and that's all they need to see. Certain people are always right!