"It is important to realize that in physics today, we have no knowledge what energy is. We do not have a picture that energy comes in little blobs of a definite amount. It is not that way" (Feynman, 1964)”

“In brief, E has no mass, because it is a calculation that uses a matter-motion term for describing the motion of matter. Matter exists, while motion occurs. Other matter-motion terms include momentum (P=mv) and force (F=ma). Neither of these actually exist, they simply describe what happens when things collide. So, Feynman is right about energy not being matter. His consternation remains today, as you still won’t get a straight answer from most regressive physicists.”

http://thescientificworldview.blogspot.com/2022/11/does-energy-have-mass.html?m=1