How was construction of the Death Star(s) kept secret?

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.