[A]n unseen ‘mirror world’ of particles that interacts with our world only via gravity might be the key to solving a major puzzle in cosmology today – the Hubble constant problem. The Hubble constant is the current rate of expansion of the universe. Predictions for this rate — from cosmology’s standard model — are significantly slower than the rate found by our most precise local measurements. This discrepancy is one that many cosmologists have been attempting to solve by changing our current cosmological model. The challenge is to do so without ruining the agreement between standard model predictions and many...