So this is really just for fun. I often talk to my friend who studied some Physics degree (or similar) and he simply cannot accept the possibility it could be wrong in any way. To the point where he gets angry if I try to challenge it.

but if E=mc2, to me this implies that the speed of light itself is somehow controlling the amount of matter an object has. It seems mindboggling to me that it's a coincidence in nature, like what does the speed of light have to do with massive something is. 

Also , is it 100% proven that in ALL circumstances/scenarios mass and energy values are directly linked, or have there been exceptions.

Finally (and off topic) that double slit test seems to me as prove that we don't understand even electrons behaviour fully (the conclusion that it will behave differently when it is being observed to when not observed doesn't sit well with me LOL!)

I'd love to hear any facts, opinions or discussion here. And of course would love to learn more and understand why this formula (and in fact the double slit thing) is so widely accepted by all experts.

(EDIT: THANKS FOR ALL COMMENTS AND ANSWERS)