Removing the CC from the normal form of the Einstein Field Equations was what he considered his blunder.
It was there originally, but he realized that he could not have a static universe with a nonzero CC, so he removed it, as Hubble, Lemaitre and Friedmann had not yet demonstrated the universe is non-static. When there was overwhelming evidence of what we now call the Hubble flow, he realized that a small positive cosmological constant produces exactly that, and thus he put it back in.
What you don't usually learn in those articles about dark energy is that in the Friedman-Lemaitre-Robertson-Walker model of the standard cosmology, you have an assortment of matter fields which are characterized by density and pressure.
Matter (in the most general sense of non-gravitational field content) has positive pressure, and some matter can clump (leading to non-uniform densities, and where density is higher, so is pressure; super-dense massive objects have enormous positive internal pressure).
Dark energy is in its simplest form a field with slightly negative pressure, and with constant density (i.e., it does not clump and it does not dilute away with the expansion like the matter fields do). This constant density is the "cosmological constant". Its absolute value is very small compared to the pressure even in slight overdensities of ordinary matter (like in sparse gas and dust clouds), so it's drowned out entirely by the positive pressures in structures like galaxies or stars.
Pressure and density are terms which are in the (Robertson-Walker) metric, and the metric describes the 4-lengths of spacetime intervals. Positive pressure contracts these lengths; negative pressure increases them.
So when fields with nonzero pressure are treated as the principal generators of the metric (i.e., matter and dark energy tell spacetime how to curve), you can call the result the metric expansion (or contraction) of space.
It was there originally, but he realized that he could not have a static universe with a nonzero CC, so he removed it, as Hubble, Lemaitre and Friedmann had not yet demonstrated the universe is non-static. When there was overwhelming evidence of what we now call the Hubble flow, he realized that a small positive cosmological constant produces exactly that, and thus he put it back in.
What you don't usually learn in those articles about dark energy is that in the Friedman-Lemaitre-Robertson-Walker model of the standard cosmology, you have an assortment of matter fields which are characterized by density and pressure.
Matter (in the most general sense of non-gravitational field content) has positive pressure, and some matter can clump (leading to non-uniform densities, and where density is higher, so is pressure; super-dense massive objects have enormous positive internal pressure).
Dark energy is in its simplest form a field with slightly negative pressure, and with constant density (i.e., it does not clump and it does not dilute away with the expansion like the matter fields do). This constant density is the "cosmological constant". Its absolute value is very small compared to the pressure even in slight overdensities of ordinary matter (like in sparse gas and dust clouds), so it's drowned out entirely by the positive pressures in structures like galaxies or stars.
Pressure and density are terms which are in the (Robertson-Walker) metric, and the metric describes the 4-lengths of spacetime intervals. Positive pressure contracts these lengths; negative pressure increases them.
So when fields with nonzero pressure are treated as the principal generators of the metric (i.e., matter and dark energy tell spacetime how to curve), you can call the result the metric expansion (or contraction) of space.