How Do Photons Have Mass?

Gluons are massless, just like the photon. But they have energy to them right? And so they move at the speed of light, just like photons do. Mass comes from internal stored energy and a photon doesn't have any internal stored energy. So somehow ther's a property of internal stored energy that it has inertia r that energy takes a force to move it around. I wish we hadl a deep, fundamental understanding of why that is, but just something we sort of observe about our universe and describe.