What mathematical evidence can you provide to support the assertion that the universe is electrically neutral on large scales? ryden, barbara. introduction to cosmology (p. 47). cambridge university press. kindle edition.

It is likely that until today there is no physical / mathematical argument that allows to equate this statement. But there are countless observations that have led to that conclusion.

From these observations two types of highlights can be summarized: Magnetic fields and Gravity Force.

Electromagnetic Field: If there is a negative or positive charge, these charges should be observed through the electric and magnetic fields by means of microwaves. However, there is currently no observation that supports any electromagnetic charge in space.

Force of gravity: The only force capable of maintaining the attraction of the planets, without affecting (at certain scales), is the force of gravity. An electromagnetic force often exceeds that of gravity, but has not been observed at the moment.