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 of curved space-time that infinitely spirals inwards towards this weird singularity where it finally converts in to mass. :uhh: But I don't believe that theory holds itself together. There must be something I am missing here. You're probably laughing at me right now, but I have no idea what mass is. I mean I do know at general level what mass is, of course, but when I try to unify my world view of vibration and mass together or even think about the process visually when energy converts in to mass, I only get blown away with multiple theories. I believe that many of you has thought about this too and I'd LOVE to hear your theories/facts.