Dark energy and dark matter are two separate things...dark matter represents up to 70% of the universe (though some conservative estimates put this at closer to 30%). Dark energy is a force that works against gravity, a repulsive instead of attractive force.<br /><br />Getting back to my original query, since there is no coefficient of drag in space (as it's empty and all) aside from gravity, which gets weaker and weaker over distance, any kinetic energy directed against an object will increase its velocity - it will never slow down until another force is exerted on it in the opposite direction.<br />This means that the accelerating expansion of the universe is occuring due to one of three reasons, or a combination of two or more: an existing force or type of energy is "pushing" against objects, as even the minuscule amount of kinetic energy in a photon can add up with trillions upon trillions of photons acting on an entire galaxy over the course of billions of years; an unknown source of gravity outside of the detectable universe (14+ billion lightyears away) exerting an increasing amount of gravitic force on galaxies, etc, as they get closed to some kind of gravitic barrier or universe event horizon; or a mysterious type of repulsive "anti-gravity", i.e. dark energy.<br />Since two of those three reasons involve known elements (kinetic energy and gravity, respectively), I'm more inclined to think along lines of a known quantity rather than inventing something else entirely to "fill in the holes". That reminds me too much of primitive peoples attributing the unexplained to a god force when they couldn't find the answers themselves. <br /><br />Of course, another alternative to this is that space itself is repulsive, that is, it exerts outward force on matter as it itself expands. Gravity may not be an attractive force...it might merely be a mass-shadow effect of matter imprinting against the outward expansionistic force of the universe. In effect, creating "shoals" in a <div class="Discussion_UserSignature"> </div>