It is an observed fact–not a postulate or a hypothesis or an article of faith or anything like that–that when we measure the distance to far-away galaxies, and then look at the light they emit, we see that they have a red shift proportional to their distance. We know from experiments on Earth (and basic physics) that a red shift can be caused by something receding from the observer.
That is the evidence, to start with. Later on, they detected radio waves from space–in all directions, any direction you look–called the cosmic background radiation–which was predicted to exist, before anyone actually detected it. It’s considered a very good sign for a hypothesis if it predicts that evidence will be found, and then that evidence is, in fact, found. (“My theory is that John Doe, having beaten the victim to death with a five iron–which is the only weapon that would make exactly the pattern of marks found on the victim’s skull–must have thrown the murder weapon into the garbage shute in the moments before the janitor, alerted by the victim’s cries for help, arrived at the scene. So, let’s all go down to the basement and look in the bottom of the garbage shaft…Aha! A five iron! With the victim’s blood and John Doe’s fingerprints on it!”)
So astronomy doesn’t look at things that are “observable”? Also, no one is claiming that the galaxy is expanding. I realize you think all this is just pointy-headed liberal secular humanist nonsense, but you really should brush up on it before you get into debates about it. Otherwise, you look like an atheist who says he doesn’t believe in Jesus Christ because he read the Iliad and there was a bunch of stuff in there that obviously isn’t true.