Electron-degenerate matter is a state of matter that occurs when electrons are packed closely together, typically found in the cores of white dwarfs. In this state, the electrons are forced into the lowest energy levels available, creating a pressure that supports the star against gravitational collapse, even without nuclear fusion. This phenomenon arises from the principles of quantum mechanics, specifically the Pauli exclusion principle, which states that no two electrons can occupy the same quantum state simultaneously. As a result, as more mass is added to a white dwarf, the electron pressure increases, leading to unique properties that distinguish it from ordinary matter.