According to various sources there were over 2,379,000 people on board the original Death Star when it exploded. On top of that it's said that it took 19+ years to build and millions of people worked on the construction.
Yet despite all of those people working on a massive project (and the 2 million personnel being transferred and living there!) people like Han Solo and Obi Wan have no idea about the Death Star. Even though Han is a smuggler who has a history with the Empire and it would be in his interests to know about it.
So my question is: How did the Empire keep the Death Star secret from the general populace and even from the underworld which would have a vested interest in that information?
In-universe answers please.