New research suggests an unseen ‘mirror world’ of particles that interacts with our world only via gravity that might be the key to solving a major puzzle in cosmology today — the Hubble constant problem. The Hubble constant is the rate of expansion of the universe today. Predictions for this rate are significantly slower than the rate found by our most precise local measurements. This discrepancy is one that many cosmologists have been trying to solve by changing our current cosmological model.