Actually, it's a theory that a "spark" in the center fuel-tank caused the downing of FT. 800. No one knows for certain that's what happened.
Many more people witnessed a streak of light than witnessed a spark.
Don't bet your life on the "if I didn't see it, it didn't happen" theory. I have been involved in designing several systems (smaller than a 747 center fuel tank) where we kept a N2 blanket in the vapor space of a vessel that contained a combustible gas. We had to do this because of past explosions that occurred when the air/fuel concentration fell below the UEL. From there, all you need is the spark and BOOM! Flooding the vapor space with N2 pushes all the O2 out. This inerting prevents explosions by removing the "air" leg of the fire triangle. Basically, the size of the spark is meaningless in a deflagration or explosion if the air/fuel mixture is between the LEL and UEL. Thus, the theory is entirely valid.