What is the most likely explanation for the dark area in the image below? 
Definitions:
Income Levels
Categories of annual income that differentiate individuals, households, or groups, often used in socioeconomic studies to analyze financial status.
Reverse Discrimination
The claim that affirmative action and similar policies intended to address inequality end up discriminating against majority or historically advantaged groups.
Meritocracy
A system where success and status in society are determined by individual talent, effort, and achievement, rather than by wealth or social class.
Affirmative Action
Policies or measures designed to increase the representation of minorities and women in areas of employment, education, and business from which they have historically been excluded.
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