@dawoel,
Historians are always making discoveries of new sources, or data, that shed light on past events; at times, these will either confirm the current account or will require modification and revision of it.
For example, the greater availability of economic data through the transference of original sources to the internet coupled with the ability to use computers to assemble and analyze large amounts of data, has provided---and will continue to do so---the basis for reassessment of our understanding of both the Medieval as well as the Victorian period (the mortality rates of different "social classes). The new studies of genetics is just beginning to make itself useful to historians (Jefferson's illegitimate children; the identification of mummies). The discovery of new documents also changes (or confirms) our historical knowledge; the release of Presidential Papers or of government archives from Communist Poland are two recent examples. Also consider how the translation of the Rosetta Stone opened up our knowledge of Ancient Egypt.
While I am not sure what is meant by "unknown facts," it seems likely that much of the prehistory of mankind will always be a matter of conjecture because of the lack of evidence of any kind. As Aristotle wrote, you cannot have a science of non-being.
As we discover new data, or improve our ability to understand it in different ways, we cannot with certainly delineate (outside of pre-history) areas that are unknowable, or "facts" we cannot in principle uncover.