List of unsolved problems in astronomy

This article is a list of notable unsolved problems in astronomy. Some of these unsolved problems in astronomy are theoretical, meaning that existing theories seem incapable of explaining a certain observed phenomenon or experimental result. The others are experimental, meaning that there is a difficulty in creating an experiment to test a proposed theory or investigate a phenomenon in greater detail. Some pertain to one-off events, unusual occurrences that have not repeated and whose causes therefore remain unclear.