I red about that because of quantum mechanics space cannot be empty. Particles and their anti particles pop into existence, and the next moment they annihilate each other and vanish out of existence. But most particles are not mass less, so this noise of short lived virtual particles; i think it must have some mass effect ? As i was reading from WMAP experiments that the average mass of the universe is close around only 5.9 protons per cubic meter. Then i'm guessing that virtual particles could easily interfere, and have a huge effect on the larger scales (the lightyears of empty space between stars). So i wonder does empty space have a mass effect, upon matter.