Here’s a fun theoretical existential threat: false vacuum decay. If our universe is in a false vacuum state (maybe yes? maybe no? we don’t know!) and that vacuum someday suddenly shifts into a true vacuum state (decays), then worst case scenario this change propagates everywhere instantaneously and we experience “complete cessation of fundamental forces” including elementary particles and structures. Everything everywhere wiped out in a blink.

We can take some comfort in the fact that we’d never know it happened, and theorists have asserted that it’s highly unlikely a false vacuum of any size could exist for more than a moment in the presence of gravitational forces, plus physics within the bubble would be “super weird”, I think is the technical term.

https://en.m.wikipedia.org/wiki/False_vacuum#Existential_thr...