Galaxies behave as though they contain far more mass than their shining stars and gas reveal. The missing ingredient is called dark matter, but the name describes a puzzle, not a substance we have identified.
What is established is its gravitational influence. Stars orbit at speeds that visible matter alone cannot explain; clusters bend background light, and maps of those distortions trace unseen mass. Across several independent observations, the evidence for extra gravitating matter is strong.
Its ingredients remain unknown. Possibilities include undiscovered particles, such as axions or weakly interacting massive particles, but no candidate has been confirmed. Dark matter appears not to emit or absorb light in the ordinary way, and observations constrain how it clusters and moves. Those clues narrow the field without naming the answer.