It's called the fossil record. We find fossils and age what we find in those rock formations that held them. At some point the fossils cease to exist(they are layered of course so this is easy to trace), an obvious reference to an extinction. We have found a layer of soot smoke and iridium that yes would have covered the entire planet. Iridium is a dead giveaway of an impact event. It's all proven science. It's called the food chain. If the sun gets blocked out plants die. Plants die plant eaters have nothing to eat...plant eaters die.....plant eaters die meat eaters have nothing to eat....meat eaters die out. Now obviously some survived(many small critters and birds, the only living descendants of the dinosaur lineage) but many of them passed away. It's not like they can run out to Wal Mart and stock up. Once the food goes they are in deep trouble. The crater for the impact was found down around Chixculub Mexico for goodness' sakes. By accident at that. That finding was the nail in the coffin for anyone who, at that time, had questions concerning an impact event being the cause of the extinction. I hope you guys are trolling. Look up Chixculub Crater. I don't see how someone can deny it unless they pretend the evidence doesn't exist. An impact event releases a lot of energy. Just look at what the small(yes small in comparison) meteor that blew up over Russia did. Imagine that actually colliding with the planet but at a width of a good 3 miles or more.
S/N there are actually people that believe the dinosaurs were the only creatures affected by the impact? Do people research this stuff at all before they do their naysaying? The answer is no obviously. You guys need to really research dinosaurs. Something on this scale would have been very tough for them. They didn't cook. They didn't store their food for later. Availability was the name of their game. But no extinction wipes out every animal across the planet. Again, this is a lack of research concerning how an extinction is defined. Some creatures have different ways of survival that aren't dependent on the sun and the food chain. Dinosaurs were not one of them.