What theory did the presence of iridium, an element only found in extraterrestrial objects, in geologic deposits dating to 66 mya around the world support?

The presence of iridium in concentrations higher than 100 times the normal in the Cretaceous-Paleogene (K-Pg) boundary supports the Alvarez Impact Hypothesis.

Explanation:

Father and Son Alvarez argued that since iridium is present in extremely minute quantities on earth, its presence in very high concentrations in this layer which is thought to date back to 66 million years is evidence that an extraterrestrial impact took place which brought the iridium. The high destructive nature of the impact spread the iridium far and wide which deposited on the surface which is why it is present in this layer which dates back to that period.