Not necessarily an infinite amount of mass. Zero-point energy is an infinite energy source. This energy is a pressure difference when any two objects have a sepperation of less than one photon length. Just like a collapsing can in cold air after being boiled, the outside pressure of the photons in the universe will push the two objects together. Although I don't think it is possible to have a sepparation of just one plank length, this phenomenon is a real source of infinite energy, energy which can be considered inert unless it is actualy in the process of being used. Therefore the source is infinite, but the mass of the energy is irrelevant because it doesn't realy exist as energy until it's in action. Also we don't have to break any laws of physics in an explanation on universal expansion. Anything that is a true property of the universe cannot possibly be a broken feature of the laws of physics. If a realistic occurence happens in the universe that seems to defy laws of physics then the laws of physics are in human error, we're the ones that wrote the books. If something is real that breaks the laws, then the books need to be rewritten. We can't tell nature it made a mistake. If at this moment the universe is creating and destroying energy then I'd say, out with the first law of thermodynamics, rather than the universe is breaking the laws of phyiscs.