How the Edge of Our Galaxy Defies Known Physics

How do we know that virtual particles have no gravitational field? The universe is allegedly full of virtual particles, and the more empty space you have the more virtual particles there are. Empty space is constantly created by the expansion of the universe and is instantly filled with virtual particles. As these particles only exist for an unmeasurably brief instant, they are allowed to borrow mass-energy from the vacuum provided they annihilate so quickly that the debt is repaid before the universe notices it's gone. But however brief their existence, there are so many of them that they must add something to the mass of the universe and therefore its gravity. Personally, I don't believe in virtual particles, but physicists assure us they are there.