Imagine the expansion rate starts out at 2. Then 1 day later it's 1.5, after 1 more day it's 1.25 and after another day it's 1.125.
Hopefully you can see that if this series continues then the expansion rate is always dropping, but it's headed towards 1, not 0. (And if expansion rate of 1 is too confusing in this context imagine if it starts out at 3 and goes to 2.5, 2.25, 2.125, ..., it still is always decreasing, but it will never be less than 2 which means the universe keeps expanding).
> What's the mechanism that allows the acceleration to drop, without dropping to zero though?
The expansion rate drops because the energy density goes down due to the expansion. However, the dark energy density aka cosmological constant does not go down, therefore providing a nonzero floor.
> Is the dark matter not expanding with everything else
This has nothing to do with dark matter in particular, and it’s space itself that’s expanding, not matter. Also, the expansion has no center, the universe is expanding at every point. Some types of matter expanding and others not would imply a center.
> The expansion rate drops because the energy density goes down due to the expansion.
I thought the idea was that Dark Energy increased with the amount of empty space; so as the space between galaxies expands, the energy driving the expansion also expands - hence the acceleration.
I wrote “density”, i.e. the amount of dark energy remains constant per unit volume. Since the volume increases due to the expansion, the total amount of dark energy increases accordingly, but its density remains the same. This is as opposed to the matter and radiation density, which decreases.
Thank you for clarifying (you said "energy density goes down due to the expansion").
Can you also clarify why galaxies don't expand, but empty space does? I suppose this is something to do with "vacuum energy", but it's not obvious to me that vacuum energy actually requires a vacuum; I thought it was present everywhere, but was only significant in the absence of other "stuff".
I also understand vacuum energy to be related to Hawking Radiation, which is black-body. But black-body radiation is EM radiation; DE is neither black-body nor electromagnetic. Why does vacuum energy not produce observable EM radiation? Is it just too weak to observe?
> you said "energy density goes down due to the expansion"
Yes, the overall energy density (dark energy plus matter/radiation) goes down, but since the dark-energy part of the density remains constant, it provides a floor.
> Can you also clarify why galaxies don't expand, but empty space does?
Gravity. Within a certain range (the size of small galaxy clusters), gravity dominates.
> I suppose this is something to do with "vacuum energy"
Vacuum energy is predicted by quantum mechanics and contributes to (or entirely constitutes) the cosmological constant, and thus is a candidate explanation for dark energy.
Hopefully you can see that if this series continues then the expansion rate is always dropping, but it's headed towards 1, not 0. (And if expansion rate of 1 is too confusing in this context imagine if it starts out at 3 and goes to 2.5, 2.25, 2.125, ..., it still is always decreasing, but it will never be less than 2 which means the universe keeps expanding).