The existence of dreadnoughts was not a secret to the upper echelon of human civilization.
However, the Common Fleet Alliance had kept them secret for so long that most people never knew about their existence.
At most, they heard rumors that the fleeters had once invested in the development of warships that were over 12 kilometers long.
To be honest, battleships around this size already became far too sluggish and cumbersome to make them worthwhile enough to deploy.
Their mass had increased to such an enormous extent that even the strongest thrusters couldn't propel them fast enough to meet the minimum maneuverability standards of the CFA.
Their other disadvantages also became more pronounced. The fleeters eventually figured out that it was much more efficient to construct two 6-kilometer long battleships in the place of a single 12-kilometer long abomination.
The fleeters never gave up on building bigger warships, though.