False vacuum

A false vacuum is an idea from theoretical physics: In quantum field theory, such a vacuum might exist for a very long time, before it changes its state. Much like with a true vacuum, a false vacuum exists where the energy is minimal. In the case of a true vacuum, this is at a global minimum of the energy function. With a false vacuum, the energy function is only at a local minimum. As is illustrated in the figure, there is a "peak" between the local minimum and the global minimum of the energy function.
Bubble nucleation.
How the system changes its energy state is called bubble nucleation: Somewhere in the universe, a bubble will appear, with a lower energy state inside the bubble. This bubble will then spread and change the energy state of its surroundings.
All energy fields are thought to be in a true vacuum state except one, the Higgs field. The Higgs field is thought to be Metastable, where it is only in its local minimum energy and is not in a true vacuum state yet.
Vacuum decay.
Vacuum decay is a theory that, should the Higgs field release its potential energy, it will create a cascade effect where more energy is released over and over again, releasing even more energy, creating a sphere that destroys everything it interacts with.
This would be completely undetectable, and some may exist already (though very unlikely. Close to impossible) Even so, they might never reach Earth due to the expansion of the universe. Current calculations suggest that if this occurs, it will be in trillions of years. By that point, the universe will be going through Heat Death.