Disclaimer: I know enough about astronomy to know that I know pretty much nothing.
As dark energy was explained to me, it is a placeholder in the equation(s) for measuring the expansion of the universe. Rephrasing, we know the universe is expanding but we can’t account for some amount of the force involved.
I hope I am making sense and I am not too far out in the weeds.
To my question: all of the stars are blasting out not just photons but also substantial amounts of physical matter in various states (gas, plasma, solid) that also includes material from the various objects in the solar system (eg atoms of water from mars). Wouldn’t that mixture of massless photons and physical material have some significant influence on everything else?
We think we have accounted for all the ordinary energy in stars, galaxies, intergalactic dust, etc., but things at the very largest scales we can observe, galaxies seem to be drifting apart from each other. Our model of physics tells us that the energy (which is also mass; e=mc^2 mass and energy are equivalent ‘things,’ just in different forms) we’re familiar with is gravitationally attractive, so we surmise there must be some sort of exotic repulsive energy counteracting the attractive stuff we’ve accounted for. And if we use our model to estimate how much of this so-called ‘dark energy’ there is pushing the galaxies away from each other (based on things like galactic mass, apparent rates of recession, and so on), we get that there is something like 15 times more of this ‘stuff’ pushing things apart than there are actual things. We dont know what this exotic energy might be, we cannot confirm our hypothesis because no way of detecting this stuff in a lab has ever been devised. It’s entirely possible our model is simply flawed, or that our observations are incorrect or incomplete.