"Is the Universe Actually Made of Math?"

Or is this just a new twist on the philosophical doctrine of "idealism"?

(Perhaps I'm misunderstanding Tegmark's thesis, but I must confess that I have no idea how mathematical ideas can be the underlying substrate of reality, with physical beings and thinking minds arising from them. That seems like a total inversion of the dependence relationship, since one would think that ideas can only exist if they are generated by a conscious mind or minds.)