As far as I know the concept of multiuniverse had been proposed by a great phisic to explain the famous Schrodiger's paradox
Is the cat in the box dead or alive ?
However it was a...joke
A journalist took it ( or pretentded to take it) as a serious theory and that's it
An other matter is the theory of the string or the brane
This theory is based on a solid math backgroud but no lab (neither common sense ) evidence