In fact, the explosion analogy is misleading because conditions were far more extreme than it implies -- the entire Universe squeezed into a volume less than a single proton, for example. In addition, the era of inflation erased many of the signatures of the conditions -- in fact, the inflation theory was invented just to explain the lack of such clues. Thus, as we go back closer and closer to the beginning, our normal procedures of scientific theorizing and testing become severely challenged, and we become more and more dependent on only partially tested theories.

From Sidney Harris, http://www.sciencecartoonsplus.com/originals2.html, see also http://astro.wsu.edu/worthey/astro/html/lec-cartoons.html