The early universe's expansion was not accelerating, as the early universe was not dominated by dark energy. I don't know off the top of my head exactly when that changed, but z=0.76 seems about right. So I'm pretty sure that is what that is.

The reason the dominant form of matter changes is fairly simple. As the universe expands, the amount of matter doesn't change, so the matter density goes as 1/r^3. The amount or radiation goes as 1/r^4 as there is an extra loss of energy due to redshifting (E = hc/wavelength). Dark energy though is (we think) a constant, so as the universe expands, the amount of it stays constant.

The gap is part of the universe we have trouble seeing because it is obscured by Milky Way.