The theory of the Big Bang states that the universe started in an extremely dense and hot state, and that it has been expanding and cooling since that moment. However, one of the misconceptions about the Big Bang is that it happened at a specific point in the universe, and that, just as an explosion, galaxies have been moving away from that point ever since.
In fact, the Big Bang took place in all points of the universe at the same time: the Big Bang didn’t only represent the creation of all the matter in the universe, but it also symbolised the creation of space and time.
Before the Big Bang space didn’t exist, so it doesn’t have any sense to speak about the point at which all was initiated. So, all the places in the cosmos that we observe nowadays come from the creation of space at the moment of the Big Bang, for which we can say that this phenomenon happened in all the points of the universe equally.
This also means that the universe doesn’t have any centre, since there is no central point from which all galaxies move away, as if it were the explosion of a bomb. The expansion of the universe makes all galaxies move away from each other, no matter which point we take as a reference.
This phenomenon can be easily understood using a balloon as an analogy. Imagine that we take a balloon and we draw some dots on its surface, which represent our universe. When we blow up the balloon, we see that all dots move away from each other, but there isn’t any centre in which the expansion started. (It is important to mention that the space above and under the balloon’s surface doesn’t have any physical sense, because it is found outside the universe).
Moreover, the fact that the Big Bang happened in all the points of the universe is confirmed by the leftover heat from the Big Bang, the called cosmic microwave background. This cosmic background is evenly distributed through all space, instead of being concentrated in a particular region.