I asked this in "lighting dark matter" but think I over-complicated the question, so here it is bite-sized...
Stars have mass, and have gravitational effect in spacetime. When their mass in converted to energy (and rightly or wrongly I consider mass just to be extremely concentrated energy), does the electromagnetic radiation then exhibit no gravitational pull, albeit tiny per unit energy?
If you consider all the - let's call it light, but I unclude other forms of EM - which has been produced from stellar mass over billions of years, there would be enormous amounts of energy ambient in the Universe. I.e. where has all the mass of all those stars over all that time gone?
If the gravitational pull of mass is lost when it's transferred to energy in stellar reactions, then would the Universe have less net gravitational force over time? (there is far less mass in stars now than there would have been 11 billion years ago).
If the gravitational pull is not lost but made much much harder to measure, wouldn't there then be a significant "background" gravitation attributable to all that lost stellar mass which is now ambient as EM radiation?