The newly proposed phenomenological mirror-matter model (M3) has just been developed into a full-fledged theory – extended Standard Model with Mirror Matter (SM3). Dark energy is simply understood as the leftover vacuum energy due to the spontaneous mirror symmetry breaking.
Here is the link: https://arxiv.org/abs/1908.11838
Many new insights: why neutrinos are Dirac particles not of Majorana type? Why neutrinos have so tiny masses? Why the lightest neutrino is not massless? Why charged leptons (electrons, muons, and tauons) have similar masses like the down-type quarks? Why vacuum expectation value, top quark mass, and Higgs mass have such a simple relation of v2=2m2t=4m2h ?
Underlying principles of SM3 : We live in a maximally interacting, supersymmetric, and mirrored world.
A perfectly imperfect mirror symmetry is indeed the key to unlock the beauty and elegance of our universe.