Where was the Big Bang?
It is all in the dimensions!
As you blow up the balloon and the surface stretches and expands, the dots get further apart. Choose a dot and watch the dots around it. Then choose another and do the same. It doesn't matter where you observe on the balloon, it will always look about the same.
This is like the Universe. The galaxies move apart as it expands. It doesn't matter which galaxy you are in, the galaxies around you move further away over time.
However, remember the surface of the balloon only has 2 dimensions. This means the centre of the "ballooniverse" is not part of it - it is outside it in another dimension!
Although our Universe has 3 dimensions, its "centre" is similarly meaningless. You would need to be able to "see" in more than 3 dimensions to be able to find it!
Do not worry if this is a bit confusing. We think in 3 dimensions. It is almost impossible for us to imagine things which are more complicated!
Scientists use maths to describe the Universe. They use these mathematical descriptions or models to predict what they should be able to see in the real Universe. They test how well the models work by observing the Universe using telescopes and other instruments. If the models and the observations match up, they know it is a good description. If the model doesn't pass, then scientists keep testing and testing until they come up with something better!