Why do astronomers think that the Milky Way is slightly barred? What evidence is there?
Astronomers carefully map locations and distances of stars, gas and dust (using all wavelengths of light); furthermore they can examine stellar orbits, which give information about where mass is located. Using these data they can reconstruct the shape of the Milky Way. Infrared information is especially useful, since in IR you can see through the gas and dust of the central bulge. The evidence points to the presence of a bit of a bar: here’s an article about it that I found.
