The single most common obstacle to understanding cosmology is probably the picture of expansion as an explosion outwards into empty space from some central point. It's a mistaken picture instilled in the public mind by popular media channels and reinforced by time-honored slang: the misleading epithet "big bang". The Scientific American magazine has a great article "Misconceptions about the Big Bang" that gets regularly recommended. A lot of us have found it helpful. http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf Here's a sample illustration.