Ok this is a difficult question but i'll try to keep it simple and if I say anything that is wrong, if it doesnt change the basis of my question or the answer then just ignore it. My question is related to particles that are given off by stars, and perhaps other places. Light is everywhere in space, stars give off light in all directions, everysquare inch of space is filled with millions of particles. If not then there would be points in space that when you were inside of them you would look around and see nothing. Since you can always see stars then there is always light. So my question is why doesnt the neverending stream of particles that fill this universe add to its weight or energy, or does it? I know that photons and neutrinos have a very very small mass but surely in the numbers that they exist in in the universe the overall weight has to be substantial. Like plankton, they are one of the smallest living things on earth but in terms of total weight they are one of the largest. Also if you can always see something so there is always light, then the entire universe is filled with light, so can there be a limit on how much light can be crammed into our universe, or does expansion solve that? I hope you all get my main point and can provide some insight on the subject.