What is a word that means "created out of nothing"

There’s always spontaneous generation, which sounds a bit wacky until someone starts chatting you up about vacuum energy with particle pairs blinking into existence and quickly annihilating each other back into the nothingness whence they came.

Ok fine, so it still sounds wacky.

The English idiom that may come closest to ex nihilo is out of thin air. But we'd need more context to decide whether it fits your purpose.

There really is nothing wrong with using ex nihilo. It is as much a part of English as et cetera is.

formal
out of nothing:
he went on to create a paradise ex nihilo

While I was hoping that there would be a word incorporating genesis, it does not appear to exist. Other less ideal candidates that you can consider are conjure and magick.