That our universe is expanding has been known for almost a century. Astronomers observed the light from distant galaxies to be stretched towards the red end of the electromagnetic spectrum (or, redshifted), indicating that these galaxies are moving away from us. In fact, the further away galaxies are, the greater the redshift of their light and so the faster they must be moving. However, seeing galaxies receding in every direction does not mean that we occupy a privileged position in the centre of the universe. Rather, it means that all galaxies are moving away from each other, because the space between them is stretching. Note that this expansion does not apply within clusters of galaxies, like our Local Group: the Milky Way, Andromeda, and a handful of smaller galaxies, which are close enough to each other to be gravitationally bound and thus able to resist the expansion of space.
