Some scientists think the material thrown into orbit originally formed two moons that later merged to form the single moon we know today. The Theia hypothesis also explains why Earth's core is larger than would be expected for a body its size: according to the hypothesis, Theia's core and mantle mixed with Earth's.
Additional evidence published in 2019 suggests that Theia may have formed in the outer solar system rather than the inner solar system, thus making it analogous to a Kuiper-Belt object like Pluto, and that much of Earth's water originated on Theia.
Theia was named for the titanessTheia, who in Greek mythology was the Titaness of Clear Sight and she gave gold its lustre and diamonds their sparkle and she was also the mother of Selene, the goddess of the moon, and Helios was the God of the Sun. which parallels the planet Theia's collision with the early Earth that is theorized to have created the Moon.
Theia is thought to have orbited in the L4 or L5 configuration presented by the Earth–Sun system, where it would tend to remain. In that case, it would have grown, potentially to a size comparable to Mars. Gravitational perturbations by Venus could have eventually put it onto a collision course with the early Earth. 
Animation of collision event between Earth (blue) and Theia (black), forming the Moon (red->grey). Celestial bodies are not to scale.
According to the giant-impact hypothesis, Theia orbited the Sun, nearly along the orbit of the proto-Earth, by staying close to one or the other of the Sun–Earth system's two more stable Lagrangian points (i.e. either L4 or L5).
Theia was eventually perturbed away from that relationship by the gravitational influence of Jupiter and/or Venus, resulting in a collision between Theia and Earth.
Computer simulations suggest that Theia was traveling no faster than 4 km/s (8,900 mph) when it struck Earth at an estimated 45 degree angle.
Originally, the hypothesis supposed that Theia had struck Earth with a glancing blow and ejected many pieces of both the proto-Earth and Theia, those pieces either forming one body that became the Moon or forming two moons that eventually merged to form the Moon. Such accounts assumed that if Theia had struck the proto-Earth head-on both planets would have been destroyed, creating a short-lived second asteroid belt between the orbits of Venus and Mars.
In contrast, evidence published in January 2016 suggests that the impact was indeed a head-on collision and that Theia's remains can be found in both the Earth and the Moon.
^Budde, Gerrit; Burkhardt, Christoph; Kleine, Thorsten (2019-05-20). "Molybdenum isotopic evidence for the late accretion of outer Solar System material to Earth". Nature Astronomy. 3 (8): 736–741. doi:10.1038/s41550-019-0779-y. ISSN2397-3366.
^Young, E. D.; Kohl, I. E.; Warren, P. H.; Rubie, D. C.; Jacobson, S. A.; Morbidelli, A. (28 January 2016). "Oxygen isotopic evidence for vigorous mixing during the Moon-forming giant impact". Science. 351 (6272): 493–496. arXiv:1603.04536. doi:10.1126/science.aad0525. PMID26823426.