As well as stars and planets, astronomers had noted fuzzy patches of light on the night sky, which they called nebulae. Some astronomers thought these could be distant galaxies. It was only in the 1920's that the American astronomer Hubble established that some of these nebulae were indeed distant galaxies comparable in size to our own Milky Way. Hubble also made the remarkable discovery that these galaxies seemed to be moving away from us, with a speed proportional to their distance from us. In fact, Einstein might have predicted that the Universe is expanding after he first proposed his theory in 1915.
Matter tends to fall together under gravity so it was impossible to have a static universe.
However, Einstein realised he could introduce a arbitrary constant into his mathematical equations, which could balance the gravitational force and keep the galaxies apart. This became known as the cosmological constant. The Russian mathematician and meteorologist Friedmann had realised in 1917 that Einstein equations could describe an expanding universe. This solution implied that the Universe had been born at one moment, about ten thousand million years ago in the past and the galaxies were still travelling away from us after that initial burst. All the matter, indeed the Universe itself, was created at just one instant.