Are gravitational effects from outside the solar system detectable?
Yes, in a couple of ways.
First we detect gravitational effects by looking a how objects we observe in the area react. If you see a bunch of visible objects moving in a certain way, you can make deductions about what is aroung. (There is some mystery around the issue of "black matter" in which they are seeing effects from gravity that imply that there is something there we can not see).
We even make testable predictions about what we expect to measure from other galaxies with our understanding of gravity. In this case it involves looking at light waves and comparing it to what we expect to see. It is pretty cool stuff.
Recently there has been research on gravitational waves. I must admit that other than a broad overview as part of coursework, I don't know much about these... other than work is being done detecting them.
What I'm thinking is that the earth isn't a single object.It is a composite of atoms etc.Likewise a distant galaxy is a composite and some look like single objects to us (am I right?).Is it the case that the atoms and molecules are what exert gravity and that they themselves have these forces?
Absolutely correct. This was one of the brilliant accomplishments of Isaac Newton. He was able to use calculus (which he invented (contemporarily with another who disputed his claim)) to simplify the problem.
Physics simplifies natural phenomina into mathematical models that can accurately predict how objects will act. In the example of the Earth, the model involves calculating the strength of gravity by using the distance from the point at the center of the earth and the mass of the earth (ignoring its size).
Newton showed mathematically why this model gives the exact same answer as if you figured out the gravitation force for all the single atoms. He took a complex interaction and reduced it to a simple one.
This technique is powerful because it works. We can launch a satellite and predict its orbit, how much fuel we need and how long it will take to go around each time. All of these calculations are based on Newton's simplification-- you don't have to know every single atom... just the distance from the center of the group of them, and the mass.
Ironically the point of all of this is that this is an example where the complexity can be reduced.
I don't know of the Copernicus quote you are referring to. I would be interested if you could find it.