Learning what fire actually is, like what the flames are literally made of, blew my freaking mind years ago.
Fire is literally just gas (and fine solid particulates suspended therein) that contains so much thermal energy that its black body radiosity has reached into the visual spectrum.
There are some types of flames that are NOT bright enough to be visible under ambient lighting conditions and can only be seen in environments that are very dark!
Pure ethanol, for instance, combusts dark enough that you can’t see the flames in daylight conditions, or even under strong artificial lighting.