A few centuries ago, everyone believed that the Earth was the centre of the universe and that everything orbited around us. This vision of the universe changed thanks to Copernicus, and it has not stopped evolving since then.
Nowadays we know that the universe has no centre. According to the standard theories of cosmology, the universe started with the Big Bang about 14 billion years ago and has continued to expand since that moment.
The Big Bang shouldn’t be seen as the explosion of a bomb, for example, which takes place in one point (the centre) and expands around it. The Big Bang wasn’t an explosion in space, but rather an explosion of space. As we understand this event today, before the Big Bang there was no space or time, so there could not have been a central point from which to expand: the Big Bang represented the creation of time and space.
The expansion of the universe is in fact the creation of space where there was none before, which causes the galaxies to move away the ones from the others at an ever-increasing speed.
The analogy of a balloon
A very common way to represent all we have just explained is by making a comparison with a balloon.
Imagine we have a balloon on which we paint some dots on its surface, which represents our universe. When we start inflating the balloon, we see that the latter begins to expand, and that the dots move away from each other. However, we see there is no centre of the expansion, there is no place that started expanding before the others. (Remember that the balloon’s surface represents our universe, and that anything above or below it has no physical meaning, as it only has two dimensions).
Moreover, we also observe that, although the dots move away from each other, we haven’t given them any kind of movement. The reason why the dots separate is the creation of rubber between them as the balloon expands.