How did scientists come up with the Theory of the Big Bang?

After observing stars for many years, astronomers found that we often found them to be a different size to those previously recorded. After continuous observation, we found stars were moving away from us. Then planets were looked at: same conclusion. The theory came about as such: If all these planetary objects are moving away from us, if we go back far enough in time, they must have started very close together. Maybe even at a single point. After this came theories about how all matter ended up in one single spot. The conclusion is an implosion, followed by an explosion.