[irishsultan]

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).

[benj111]

Ok, that makes sense.

What's the mechanism that allows the acceleration to drop, without dropping to zero though?

Is the dark matter not expanding with everything else

[layer8]

> 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.

[rout39574]

Yes, that is indeed the question.