Comment by codethief

> Combined with the other observation that our universe has uniform density at large scales

s/has/had at the time of recombination

It is largely an assumption of LCDM that we can treat the universe as practically homogeneous throughout its entire evolution but potentially not a very well-founded at that [0, 1].

> I always felt uncomfortable with that characterization. It seems more reasonable to me to think of dark energy as _negative energy_ - i.e. a loss of overall energy.

Your intuition is correct. If the Lambda term in the Einstein field equations is moved over to the side of the energy momentum tensor, it takes on the role of a negative contribution (provided Lambda > 0, as observations seem to indicate).

> In a classical system, two things moving away from each other stores potential energy that can be recouped at some later time. Dark energy doesn't work this way - things accelerate away from each other the further apart they are. From a global perspective, it's an energy loss.

Note that there is no global energy conservation in General Relativity[2], only at a local scale[3]. Heck, you'll already struggle to define what the energy is of a given piece of spacetime in a meaningful and generic manner[4, 5]. In other words, violations of energy conservation due to spacetime expanding or contracting (a strictly non-local phenomenon), like in the case of the cosmic redshift, are expected and our intuition from classical mechanics only takes you so far.

> It somehow feels more appropriate to me to think of dark energy as energy being extracted out of the universe, in some form never to return.

Dark energy aka the cosmological constant term in the Einstein field equations is a constant term, as the name suggests. Yes, there can be energy loss due to spacetime expanding (see above) but that doesn't change the gravitational constant.

[0]: https://en.wikipedia.org/wiki/Cosmic_web

[1]: https://en.m.wikipedia.org/wiki/Inhomogeneous_cosmology

[2]: https://en.m.wikipedia.org/wiki/Conservation_of_energy

[3]: https://en.m.wikipedia.org/wiki/Stress%E2%80%93energy_tensor

[4]: https://arxiv.org/abs/1510.02931

[5]: https://en.m.wikipedia.org/wiki/Mass_in_general_relativity