A Level I multiverse requires infinite space and matter and random seeds that create fluctuations as it expands.
While a finite universe works within Einstein’s theory of general relativity, recent observations constrain models that would show this. The only spaces that fit what scientists have seen have a volume comparable to or greater than our universe.
Scientists have observed signs of random seeds: there must be a mechanism that caused them. People have measured their statical properties and confirmed that the fluctuations are consistent with a “Gaussian random field”.