Here is a thought expirement. I have no data to back this, but that puts it on par with most other explanations.
As the universe expands, relativity predicted it would slow down. What we found was that it was accellerating. From this dark energy was dreamed up. What if it is actually slowing down? Here is my reasoning:
We know that the greater the gravity, the slower time passes. So really the more space is warped or stretched, the slower time will flow. So, as the universe expands space is stretched everywhere, and the rate at which time is passing would slow down with that. We are slaves to time, and so are our observations. So as we slow down, a constant rate of expansion would actually appear to us to be an accelleration.


