As far back in history as one can trace there are strong elements of mysticism and imagination that form the framework for some culture's cosmology. In spirit, our present cosmology is not too far removed from that of the ancients. While we certainly have a much better empirical understanding of what's in the Universe, our knowledge of the physics that drives its evolution remains limited. Hence, just as in ancient cosmology, today there is still much about the Universe that remains mysterious, perhaps even mystical. Indeed, as we will come to learn, we can't even answer a rather fundamental question: What is the Universe made of?
In the study of Cosmology, all we have available to us is observation since we can not (yet) physically explore areas beyond our Earth. As our observational tools have become increasingly more sophisticated, new discoveries are made which alter our Cosmology. Often times these new discoveries have occurred as the result of new instrumentation which has opened up windows into the electromagnetic spectrum that had been previously closed. For instance, all ancient cosmologies had only naked eye visual observations in which to base their models on. Today, our modern cosmological model is strongly underpinned by observations made in the microwave portion of the electromagnetic spectrum. Had new technology not allowed this observing window to become accessible, we would still be missing a crucial clue as to the overall nature of the Universe.
Still, even though we now live in a highly technical world and have wonderful cosmological instruments such as the Keck 10-m telescope or the Hubble Space Telescope , it would be extremely presumptuous to think that we have now arrived at a thorough understanding of the Universe. We are just in the beginning data acquisition stages in our knowledge of the Universe and these data have provided far more questions than answers:
In the beginning, when little was "known" about the world, cosmological models sprung from the realm of the imagination of early humans. Since that time, our models have evolved in terms of physical precision but along the way model making has been plagued by mistakes in reasoning and erroneous observations. Historical memory of the evolution of model making and the methodology which is used, gives us important lessons to be learned about our current cosmology. For instance, just as we now view the ancient notion of an earth-centered Universe as absurd, we should be reminded that this model arose from a combination of observation, bias and ignorance. Those same conditions exist today.
More specifically, as we will see, we have the unknown state and amount of dark matter at the heart of most current cosmological models. No one has yet directly detected this dark matter component to the Universe and we have little clue as to its real nature and only partial clues as to its very existence. These clues exist only in the context of the assumption that we fully understand the behavior of gravity on large scales. It is entirely possible that future physicists will have discovered the existence of another long force in the Universe that alters the way gravity works on large scales. Should this be the case, our modern dark matter dominated cosmology would join the realm of the absurdity in the same way we now view the ancient earth-centered cosmologies. This state of affairs simply reflects our primitive and naive inquiries about the nature of the Cosmos. We may not even know the right kind of questions to ask, let alone the right kinds of observations to make. As such, a state of lingering ambiguity afflicts our current generation of cosmological models, but this has always been so. The only sure way to remove ambiguity is via the acquisition of definitive data. Througout history, including today, this has always been difficult to do.