[layer8]

> 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.