Even if it's not a smooth expansion, such as one part moving slower than the rest? If that's the case, you still have the same problem, while the other atoms are going slower, then the faster ones are gaining even greater distance.
Well it is a measurable phenomena, itÃ¢â‚¬â„¢s not as if there s any debate about whatÃ¢â‚¬â„¢s is being observed i.e. the universe is expanding.
Although the term big bang leads one to draw upon an explosion as an analogy, itÃ¢â‚¬â„¢s not the best. The big bang is an expansion, the difference being there is no shock wave or the like. If one were to travel back in time to a short time after the big bang, position yourself Ã¢â‚¬Ëœhalf wayÃ¢â‚¬â„¢ from the Ã¢â‚¬ËœcentreÃ¢â‚¬â„¢, you would not experience any motion or view particles screaming past you, as if on the way to the Ã¢â‚¬ËœedgeÃ¢â‚¬â„¢. What you would see is the universe moving away (in every direction) like the zoom feature of a video camera in reverse. As far as you would be concerned you are stationary (as is every other particle) it is space that is expanding and you are along for the ride.
Besides all this, I think I'm trying to debate on unfamiliar ground, I haven't taken enough classes or read enough yet to really understand all of these concepts.
Cosmology is a non intuitive yet fascinating subject, and by no means has a conclusive answer been agreed upon, in my life time the theories have swayed one way then the other several times. You live in an exciting age of discovery regarding cosmology, and you being in your teens will see much advancement in the understanding of the fabric of the universe.