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In his Australia lectures and his six easy pieces book, Feynman unequivocally states that mass is momentum in the direction of future time.

Anything that experiences the passage of time has mass, and everything that has mass experiences time passing.

Seeing it this way makes many things simpler. For example, if you believe, as I do, that the ants-on-an-expanding-balloon model is accurate, and that the balloon is expanding into future time with the big bang in the center, then mass is naturally seen as the surface of the expanding balloon.