Right, but how do we know that a given chunk of matter has mass? It resists being moved when force is applied to it, and it exerts a gravitational pull.
But why does it resist being moved, and why does it exert a gravitational pull?
Good point. When this particle is found and identified, I assume there will be a mystery just as mysterious as to why the particle acts the way it does in order to “cause” mass?
Great question. And one the experts have been struggling to figure out for centuries.