This universal expansion represents only the average motion of the galaxies, the motion of a given galaxy can present deviations from this average. For example, galaxies which are close together are bound by their mutual gravitational pull and this distorts the Hubble flow.
The General Theory of Relativity predicts that the universe is not static, and observations confirm this indicating that it is expanding. Thus the universe must have been smaller in the past, and, following this idea to its limit, must have been a point in its inception. Thus the universe began at a point, in the distant past and has been expanding ever since. The event marking this beginning is known (with a characteristic scientific flair for words) as the Big Bang.
Just after the Big Bang the universe contained an extremely hot and dense soup of matter and energy (which are equivalent in the sense of the Special Theory of Relativity) under which conditions any kind of object would melt almost instantaeously into its components. Yet the universe expanded and cooled accordingly, and this cooling allowed for the formation of more and more complicated structures, ranging from atoms (300,000 years after the Big Bang) to Galaxies (109years after the Big Bang) (see Fig. 8.16).
It must be remembered that the Big Bang represent the creation of the universe, including space and time. The Big Bang is not to be pictured as a big explosion somewhere out in space with galaxies being spewed out from the explosion region. Instead the picture provided by General Relativity is of the whole universe, including space, appearing at the Big Bang and expanding after that (like the balloon model described above). In this picture the Big Bang occurred everywhere.