At present, our best lunar formation model involves a Mars-sized object colliding with the early Earth. Two new papers published by Science challenge this theory and propose two very different models for how our moon was formed.
A paper by R.M. Canup proposes that, instead of being considerably smaller than the Earth, the impacting body was around 80-90% of the Earth’s size.
A second paper by M. Cuk and S.T. Stewart, published in the same volume, proposes that a Mars-sized object collided with a rapidly rotating early Earth, where a day only lasted a few hours. This collision produced a debris disk that then accreted in to our moon.