Dark matter is an unidentified type of matter that makes up 27% of the observable universe, as first proposed in 1933 to explain evidence of "non-visible" mass affecting galaxy motions. It is classified as cold, warm, or hot depending on its properties, and its existence helps explain general relativity and the expansion of the universe, though more is known about what dark matter is not than what it actually is. The document provides an index and overview of the concept of dark matter, evidence for its existence, its classification, problems it poses, and bibliography.