Until well into this century astronomers did not know that the Milky Way was a galaxy and that the `island universes' seen through large telescopes were galaxies, systems of many, many stars grouped together like the Milky Way.

The fundamental discovery which demonstrated this was that made by Hubble. He showed, from spectra of the galaxies, that there was an increase in the velocity of recession with distance. The deduction from this is that space is expanding and it was soon appreciated that the Milky Way was one of a very great number of galaxies and that it, like the Sun, had no special place in the system of galaxies.

From the observation of galaxies using optical wavelengths it was not possible to find evolutionary effects and so the hypothesis that the Universe was in a steady state was a plausible one. With the advent of the large radio telescopes it was found that there were far more faint radio galaxies than one would expect in a steady-state universe. In fact it was shown that it was likely that all the galaxies originated in a very small volume -- the Big Bang.

This theory received a boost when radiation at 3 degrees K, the micro-wave background radiation, was discovered coming from all directions in space. This radiation was predicted to be a remnant from the very early time in the age of the Universe, before matter had been formed when the Universe was still filled with hot radiation. The radiation was isotropic and it corresponded to a temperature which was consistent with red-shifted radiation from the Big Bang.