Martin Harwit  has argued that we cannot have made more than ten per cent of the crucial discoveries in Astronomy. He uses what John Barrow aptly calls `the proof-readers argument'. If two independent readers look at a manuscript then it is possible to estimate, by comparing their different results, how many errors there must be in total, including those not identified. In an analogous way two independent astronomical channels (say optical and X-ray) can be used to examine the Universe and a comparison of their separate key discoveries will yield an estimate of the numbers still to be found.
In any case with so little data to work on it shouldn't be too difficult to devise a plausible theory to account for them. It is, however, sobering to compare the cosmological situation with the history of other sciences.
Take geology. Men were living on the earth for millions of years, and quarrying rock, digging mines and canals and puzzling over its fossils for thousands of years, before unexpected palaeomagnetic patterns revealed for certain the key idea of Continental Drift.
In stellar physics two thousand years elapsed between Hipparcos's speculations and Bessel's first measurement of a stellar distance. Seventy years later the statistical patterns in the H-R diagram led to our understanding of stellar structure.
However the closest comparison comes from my own field of galaxy astronomy which is, as an observational science, almost exactly contemporary with cosmology. Although we now have good spectra and images of thousands of galaxies the list of fundamental things we don't know about them (Table 3) is far more striking that the list of things we do.
|1.||How our knowledge is warped by Selection Effects.|
|2.||What they are mostly made of. (Dark Matter?)|
|3.||How they formed - and when.|
|4.||How much internal extinction they suffer from.|
|5.||What controls their global star-formation rates.|
|6.||What parts their nuclei and halos play.|
|7.||If there are genuine correlations among their global properties.|
|8.||How they keep their gas/star balances.|
Of course these are only arguments by analogy. The optimistic cosmologist can always counter argue [I don't know how] that the Universe in the large is a great deal simpler than its constituent parts.