The word "density" means mass per unit volume. It's sometimes qualified like "charge density" for (assumed, in turn, to be electric) charge per unit volume, but

The word "density" means mass per unit volume. It's sometimes qualified like "charge density" for (assumed, in turn, to be electric) charge per unit volume, but "density of charge" would mean mass per unit volume of charge, like "density of iron", means mass per unit volume of iron. So density of space is mass per unit volume of space, not space per unit volume.