A phenomenon like the Casimir force might reveal why we have unaccountable expansion of space. The Casimir force is demonstrated when two metal bars in near pure vacuum are set sub-atomically close and a bizarre force pushes them apart until atomically or more distance apart. In sum the physicists figured something appeared in the tiny space. The only thing that made sense was that particle fields exist completely, everywhere, and due to fluctuations a void or vacuum would not be truly empty but have spontaneous particles fill it as energy moved into the emptiness and manifested into particles. Each of the 17+ known (theoretically) particles would allow for a decent amount of possible particles per distance. Meaning if the bars are nearly touching a tiny particle could suddenly manifest in that space but nothing larger could because existing matter is blocking it. But tiny gaps will eventually have a particle get in, wiggle or decay, and force some more space until eventually many particles come in pushing the bars noticeably apart. This would appear as an unseen pressure from nowhere; a dark energy.
So, if some force initiates a void or vacuum it could result in dissipation of some energy into the void as a new particle, and if this cascaded it would match the cosmological constant, aka the rate of spacial expansion. The issue would lie in where do the particles go? Well maybe these vacuum-filling, spontaneous particles are equally likely to be any particle and on average annihilate as they pair up with their anti-matter counterparts. The outward burst of annihilations will leave a void and new particles can pop in allowing for a continuation and exponential growth of this phenomenon. This would likewise propel the universe toward heat death as energy is basically being converted to space. Really, the energy is conserved but it exists in a larger space over time which concludes with a energy:space ratio near zero. An extreme E=mc^2 solution with all energy equating to a universe of just space.
Another more fundamental explanation for Dark Energy would thus be energy lost from fundamental particle interactions. The weak and strong force may not perform at perfect efficiency, but any energy lost is too small to become anything else and is somehow absorbed into space-time. The accumulation of these events would mean space-time will grow and more space-time gives more opportunity for rogue tiny energies to decay into more space-time. This could account for the exponential Universal Constant we observe.