The most popular scientific understanding is that a (x) mile wide meteorite hit the earth near the Yucatan Peninsula in the Gulf of Mexico, but even this is still a contested theory and is far from fact.
inb4 birds are descendants of dinosaurs so they're not technically extinct
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My opinion: A meteor struck the Earth, it killed some dinosaurs on impact, however the remaining survivors were killed off by an ice age caused by the dust and ashes spewed into air after the collision.