Some scientists believe the universe is expanding because of the observed phenomenon called redshift. This is the stretching of light's wavelength toward the red end of the spectrum from distant galaxies, indicating that these galaxies are moving away from Earth. The farther away a galaxy is, the faster it appears to be moving away, which suggests that the space between galaxies is growing over time. This observation supports the idea that the universe itself is expanding.

This expansion was first confirmed by astronomer Edwin Hubble in 1929 through studying redshifts of spiral galaxies and their distances. The relationship found, now known as Hubble's Law, shows that objects are receding faster the further away they are. Later, observations of distant supernovae in 1998 revealed that this expansion is actually accelerating, an effect attributed to an unknown force called dark energy that acts like a repulsive force pushing galaxies apart.

In summary, the key evidence for believing in the expanding universe is the redshift of light from galaxies showing their recession, the proportionality of galaxy speed to distance (Hubble's Law), and the accelerating rate of expansion discovered through supernova observations, all of which point to an expanding cosmos.