Thank you for your answer DaleSpam :smile: This helps me quite a lot!
But when you said: "It does not "convert" from energy to mass, they are both there all the time and in the same amount."
I got blown away with the "same amount". As far as I've learned, some particles have more mass...
I've been wondering this question for a long long time.
In my world view, all energy is vibration/frequency/waves etc. and it's quite easy for me to imagine.
But then there's mass. To me it's like a compressed form of energy, where loads of energy is inside an imaginary ball or inside a path...