The Galaxy's dimensions are 100,000 light years in diameter and approximately 2,000 light years thick
And it is ever-expanding, so perhaps at the beginning it was perfectly flat, but without knowing the acceleration of each dimension how can you say it's not expanding vertically? And then how do you know it's not horizontal or which way is up in space in relation to no valid point of reference? To say it is flat is simply another way weak minded people cope with what they see. You believe it is flat, just like you believe the basic laws of physics to be true. I don't consider it to be flat. Without actual organization and measurement of where every planetary body is located in the 2,000 light years of thickness, you can't really say it's flat unless they all line up exactly IMO.
I would consider the area to be 'bumpy' as a compromise, but not flat. :)
