My theory is that dark matter is actually just a thin patina of grime covering the whole universe, and we don't notice it because we haven't thoroughly cleaned the place in eons.

"Dark Matter Candidates", by Randall Munroe, licensed under Creative Commons Attribution-NonCommercial 2.5 License

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My theory is that dark matter is actually just a thin patina of grime covering the whole universe, and we don't notice it because we haven't thoroughly cleaned the place in eons.