Khloe Kardashian & Lamar Odom -- Khloe's Dark Secret

Khloe Kardashian kept Lamar Odom's crack addiction secret from her entire family for most of her marriage ... TMZ has learned.

Sources connected to the family tell us ... Khloe did not want to burden her family and jeopardize the Kardashian machine by telling the brood about Lamar's drug issues. We're told Kourtney and Kim Kardashian learned of Lamar's addiction "very recently."

We're told Kris Jenner has known about it for a longer period of time, but was still kept in the dark for many months.

Our sources say even when Khloe reached out for professional help -- for example hiring private investigators to follow Lamar -- she lied about the extent of the addiction. Our sources say Khloe told private investigators Lamar was addicted to "performance enhancing drugs" and that's why she needed their help. She never played the crack card.

When stories started to surface about Lamar sleeping with other women ... and the family started asking questions ... Khloe felt she needed to reveal the real issue. As one source put it, "Khloe was a real trooper. She's been living a nightmare."