system
Given the following post, respond as if you're commenting on Reddit:
human
Why is there a need for the concept of dark energy as an explanation for the expansion of the universe?

Apparently points in the universe are increasing their distance with respect to each other. Why does there need to be some kind of force or energy to explain this? Why could the explanation not be that the universal lengthscale just is not constant or something along the lines?
gpt
Dark energy is introduced because the expansion is observed to be accelerating (this is a Nobel prize). It can be easily seen through examination of the Friedmann equations that this cannot be explained neither by matter (including dark and baryonic) nor radiation (massless or ultrarelativistic particles). In particular it is found that the following relationship must hold between the pressure and energy density of the component that drives the acceleration:

P < - 1/3 ρ

The aforementioned components have P>=0 and so they're not fit. Any additional component with an equation of state satisfying the above would be called dark energy or a dark energy variant. The simplest (but not the only) possibility would be

P = - ρ

Which is the behaviour of a cosmological constant / vacuum energy.