Historical clue formulations

• What few people know — 2025-01-19
• Highly specialized knowledge — 2021-06-27
• Specialized knowledge — 2015-02-13
• It's hard to understand — 2011-12-18
• Dark matter? — 2009-10-10
• Crossword topics, often — 1996-11-01
• Brain clutter — 1994-12-24