Iridium is a hard, dense, silver-looking transition metal often found in high abundance in meteorites. When the higher-than-usual amount of iridium (around 100 times greater than the average in Earth's crust) showed up in a specific rock layer known as the Cretaceous–Paleogene (K-Pg) boundary, scientists were rather puzzled.
The K-Pg boundary is the geological signature of Cretaceous and Paleogene periods, dated back to about 66 million years ago. A massive extinction event marks this boundary: all non-avian dinosaurs and 70% of all other species at the time went extinct, and some became fossilized and was later found at the K-Pg boundary.
A diverse group of scientists, physicist Luis Alvarez, his son, geologist Walter Alvarez, and chemists Frank Asaro and Helen Vaughn Michel, was first to propose to the Alvarez hypothesis, suggesting that an impact of a massive extraterrestrial object, for example, a meteorite, caused the extinction of the majority of dinosaurs.
Even though others argued that volcano eruptions might have released a lot of iridium instead, there is no doubt that a meteorite must have landed on Earth and caused the death of many life forms when the Chicxulub crater, a massive point of impact on the coast of Yucatán, Mexico was identified sharing the similar geological time signature with the Iridium Anomaly.
Source: SciShow via Youtube