Five thousand years ago there may be many gaps in the historical record, fifty years ago there may be so much information that it is virtually impossible for a single historian to digest the material.
Physical evidence from the past may provide firm evidence for what "really happened". Thus an archaeological dig of a battlefield may reveal a wealth of information about the type ammunition used, the number of dead and even the type of food that the armies were eating. However, even with a wealth of physical evidence, the historian's task is to place it in context. Thus, what does the type of food that was eaten by an army mean to the overall reality of what was occurring at a time
Another difficulty with discovering "what really happened" is that the historian needs to decide from whose viewpoint are the occurrences being seen. The traditional, "great man" view of history, which tells of the happenings that occurred to Kings, Queens, Emperors, Presidents, Prime Ministers . . . . that is very different from a history of the ordinary or poor people. The latter may be virtually unaware of what is happening at the national level, while the former may ignore the plight of most of the people that they rule. So "what really happened" depends upon the point of view being taken. "Happening" is a multiple occurrence and has multiple dimensions according to the different groups and individuals being considered.