The Moon travels round the Earth. The Moon moves very fast. It would fly off into space if it was not pulled towards the Earth by gravity. Gravity is like the fishing line that stops the sinker from flying off into the lake.
There is just the right amount of gravity-pull to keep the Moon going round the Earth: if there was less gravity the Moon would fly off into space: too much gravity and the Moon would be pulled right into the Earth. That would be another kind of big bang!
