Mirror symmetry, understood as local orientation symmetry, has been conjectured to allow two sectors of particles in our $4D$ universe. This idea is further developed by considering dimensional transitions of spacetime during the inflating early universe. Based on mathematical results from string theory, supersymmetric mirror models are constructed for spacetime of different dimensions to understand the early universe and black hole physics. Under $4D$, dark matter is just ``mirror matter'' while dark energy originates in the nearly-canceled residual out of two gauge vacuum energies of the two sectors. The new framework is particularly intriguing as laboratory experiments are ready to test its unique prediction of neutral particle oscillations. This is a short version of https://osf.io/37kb8 written for the Gravity Research Foundation 2023 Awards for Essays on Gravitation.