What came before the big bang?

While the big bang marked the beginning of space and time, physicists are developing testable theories about what might have preceded it. Three leading proposals include Hawking and Hartle's "no-boundary" model where the universe forms a closed four-dimensional surface with no true beginning, Steinhardt's cyclic universe that slowly contracts then rapidly re-expands in endless cycles, and Boyle's mirror universe theory where time flows backward before the big bang with antimatter dominating.