Matter is energy...so enregy adds to the gravitational attraction of the univerese.
Some theories of the Big Bang postulate that there once must have been equal amounts of matter and anti-mattter, but the anti-matter all got destoyed by interacting with amtter, creating energy, in a rather rare sub-atomic transatction. Only a small fration of the original matter survived, and even less of the anti-matter. So what happened to all that excess energy, and why wasn't its gravitation force sufficent to stop the Big Bang in its tracks and lead to a quick collapse of the budding universes? Wasn't that a tremendous amount of extra mass, now undetecable because it is lost in the BackGround radiation?