Full Idea
Some philosophers deny there could have been anything other than what in fact exists, or that anything that exists could have failed to exist. This is developed in very different ways by Wittgenstein (in 'Tractatus'), Lewis and Williamson.
Gist of Idea
Some say what exists must do so, and nothing else could possible exist
Source
Robert C. Stalnaker (Mere Possibilities [2012], 1)
Book Reference
Stalnaker,Robert C.: 'Mere Possibilities' [Princeton 2012], p.1
A Reaction
This could come in various strengths. A weak version would say that, empirically, that all talk of what doesn't exist is vacuous. A strong necessity (Williamson?) that totally rules out other possible existence is a very odd view.