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?
According to Einstein's E=mc^2, mass and energy are manifestations of one other. And since space is not really empty but rather seething with vacuum energy, matter must somehow interact with it to cause space-time warpage, aka 'gravity'. Perhaps matter, in order to exist, 'feeds' off the energy of the vacuum thereby creating regions of net 'excess' density, or excess 'pressure', in the region surrounding the object? This effect would both make it more or less difficult for objects to 'plow' through space (depending on the amount of mass they contain/vacuum energy they use up) and would also draw outside objects into the region they occupy, again depending on the amount of mass they contain.