Why do atheists think that the default is nothing if religion is false?

Techically since nothing is known about experience after death, why is nothingness the default? There is a number of infinite possibilities (entering into another universe, falling forever in a endless pit, becoming some sort of pure thought, ect. ect.) Why do atheists think the logical answer is nothingness? I don't even know what nothingness is since I have never experienced it.