What is the scariest concept in theoretical physics that most people haven’t heard of?

Vacuum decay.

My Dyson keeps breaking.

What is that?

I like quantum immortality.

Not sure, I've never heard of this.

Where a portion of empty space decays to a lower quantum state. This then propagates through the universe at the speed of light destroying all things. ( no I don’t understand it either).

Theory that the vacuum of our universe is not in its most stable state. In its most stable state it would annihilate all matter. Theory is that if a ‘bubble’ of vacuum somewhere in the universe collapsed into its most stable state, the bubble would quickly spread and annihilate all matter in the universe. Falls into the category of probably not correct since it hasn’t already happened, and if it is correct and it does happen you won’t know in time to worry about it.

Tooth decay is scarier.

Closed time like loops

The idea that the current vacuum in space is not the true vacuum or energy level. And if somewhere the a particle pops into the real vacuum there would be a cascading effect that would spread at speed of light breaking down atomic structures as we know them and we would cease to exist before we know it.

lol classic

Too many Milky Ways will do that.