Explain why it only appears that we are at the centre of expansion of the universe and why an observer in another galaxy would see the same relative motion of all but the closest galaxies away from her.

By considering the conventional point we can say that whenever materials get expanded, if will expand from the central point after the explosion. However before the big-bang there was no space and time.

The volume of space expands at a constant rate. As a result, the distance between galaxies is growing, implying that all galaxies are moving away from one another. So, from our point of view, everything is moving away from us, and it appears that we are at the centre of the expansion. All of the other galaxies are also traveling away from them. As a result, we can only conclude that the universe is expanding universally because any observer may declare that distant galaxies are moving away.

Therefore, as far as any observers can tell, distant galaxies are moving away from us, implying that there is no centre of expansion.