Here’s the wikipedia page for the theory, which makes for a pretty cool read: https://en.wikipedia.org/wiki/Giant-impact_hypothesis

Some cool stuff:

• The other body was a mars-sized protoplanet called Theia.
• According to computer simulations, remnants of Theia could be still visible inside the Earth as two giant anomalies of the Earth’s mantle.
• By examining isotope ratios in lunar samples from the Apollo missions, we can see the moon and earth likely had a common origin.