You sound just like my college physics professor. "Don't go into astronomy, its all just optics!"
Take it from me, astronomy is
much much more then just looking at spectral lines.
You want some predictions? Okay, here are some big ones:
1) Astronomers, working under the Big Bang assumption, figured that if you played everything back in time that energy would be crammed into a smaller space, that means the temperature would go up. And also since galaxies couldn’t have formed yet, we’d expect a gaseous sort of universe early on. Hot dense gasses emit photons at a peak wavelength corresponding to their temperature. Unfortunately, since things were so dense, photons couldn’t get very far.
However, with the available information, astronomers were able to determine at what density and time, photons would finally be able to get far enough that we could observe them. This is called the “surface of last scattering” and has a very specific temperature. So we should be able to look for photons with energy (wavelength) corresponding to that temperature.
But due to red shift, they will appear at a different wavelength. This radiation should appear from every direction. This was a prediction made by the Big Bang theory.
Years later, it was confirmed by the discovery of the cosmic background radiation (CBR) at exactly the predicted wavelength.
2) Observations of the behavior of galaxies suggested the existence of "Dark Matter" which did not follow the rules of traditional black body radiation. It's existence was all but confirmed a few months ago. The description is rather technical, so I'll just link to it.
http://cosmicvariance.com/2006/08/21/dark-matter-exists/3) The existence of Pluto was first postulated owing to an observed irregularity in the orbit of Uranus. After running through the math, astronomers came up with a predicted location for where Pluto should be. Percival Lowell aimed his telescope at that location, and discovered a planet (Well. . . it's not a planet anymore)
4) And then of course there are the more obvious predictions, like the eclipses and transits that astronomers regularly predict with unprecedented accuracy.
I can keep going. There are dozens more I could name.