I had a professor once give me his opinion on this, without using dark energy.
His theory was that universal expansion was not slowing down. He thought that the the universe must be spherical, or any geometric shape with a high volume to surface area ratio, and that instead of actually slowing down, the universe was still increasing at the same change in volume rate. It was the apparent motion of objects we observe that make it seem like it is slowing down.
Like making two dots side by side on a balloon and then blowing air into it at a constant rate. The dots will move away from each other much more quickly as soon as the rubber starts stretching than later when the balloon is almost full.
He was not a cosmologist, though. He was a solid state physicist working on semi conductors. But his explanation sounded a lot more elegant to me than when my astrophysics professor answered the same question by talking about some hypothetical matter that all of the sudden recently decided it wanted to act on the expansion of the universe. Although Dark Matter/Energy research has come a long way in the past 6 years, its not looking so hypothetical now.
). From your description, I would say the predicted slowing was based on gravity slowing down the relative velocities between objects and it did not take into account the cosmological constant.
