First off I believe (and I'm not alone) dark matter doesn't exist. It's part of a mathematical cheat in order to fudge an outcome we'd have no other way of explaining. What I do believe is the universe isn't speeding up. Rather we in our localized time zone (influenced by gravity) is how we measure things differently apart from the whole of the universe. This is what is making the outer reaches of the universe appear as though they are speeding up. Think of it this way. Suppose our location were at the stem of a balloon. As the balloon expanded it would appear to us other objects on the surface are increasing their speed as they draw away from our location.
These departing stars would be traveling separate apart from time as we know it within our location. The same rules regarding the speed of light could still apply to us and they both. However the two would no longer bear a relationship to one another because of the fluidity of time. Thus allowing them to appear traveling beyond the speed of light in reference to our time locality but not actually wherein the time in which they exist. This has a huge implication being some of the light from those traveling away from us may never reach us.. ever!
Our current light measurements are only able to observe stars some 13.5 billion light years out. Thus being ensnared within our localized time zone we can't rule out the possibility the universe extends far beyond only the light we are able to detect. Perhaps into infinity.
How we measure time is it's relationship between objects. In other words time's observable effects. While this is suitable for our everyday needs it falls far short of explaining what time in-of-itself is. Without true comprehension of how time is manufactured or if it even exist we can never hope to understand the mechanisms of our universe. Every measurement of ever hoping to understand our observable universe depends on our knowing. Yet it remains perhaps the greatest mystery of all.
Universe Expanding At An Accelerating Rate, Or Is It?
Is time speeding up due to the inflation of the universe?