The current model of lunar formation grew out of the data from the lunar missions, in particular, the isotope ratios of the lunar soil samples as compared with those of the Earth.
In more recent years, a couple of problems arose; the math shows that the composition of the formed Moon would be about half from the proto-Earth and half from the impactor. The isotope ratios are not an exact match, but none of the known isotope ratiios from the rest of the Solar System would provide the needed match.
The second problem is obscure, but is my favorite -- that the fossil nautiloids preserve the major septa, showing the lunar month length over a couple hundred million years. There was a nice slow lengthening of the month, but the Moon was far too close at the beginning of the fossil sequence to permit an impact origin of the Moon -- the Moon has to have been formed elsewhere and captured. Since there's no way around the fossil evidence, the non-exact / pretty close match of the isotope ratios loses significance quickly.
Captured? I wonder where it came from.
Had the moon been captured, why are we losing it? Well, after zillions of meteoric impacts, I guess the moon IS losing weight. (And so is Earth).