Not sure if I'd call this a conspiracy. Some certainly would, others not so.
There's an increasing amount of compelling evidence that there may have been an advanced human civilisation thousands of years before the accepted time line.
There are monolithic sites which have been found that predate the believed building of the pyramids by many thousands of years at a time when man was supposed to be no more than primitive hunter/gatherers.
Further to this, there is a lot of compelling evidence of a large scale global cataclysm around the time that may have 'pressed the reset button' on man kinds progress. The most popular theory is an arctic meteor strike causing global flooding (a possible source for the great flood myths found in many religions and cultures today)
Of course, the loons (Ancient Aliens, Atlantis believers etc,) are all over this, but there is some genuinely compelling evidence coming through.
There is a lot of resistance from a lot of historians though and subsequent mudslinging. Some historians/researchers saying this is 'psuedo-science' and the other side accusing the academics being ignorant in order to protect their careers.