To explain how atoms were overall neutral, he proposed the “plum pudding model” in 1904. He suggested that the atom consisted of negatively charged electrons randomly distributed in a positive ...

That could only happen if there were a knot of electric charge at the center of an atom, rather than being spread out as in the plum pudding model.

Everyone at that time imagined the atom as a "plum pudding." That is, it was roughly the same consistency throughout, with negatively-charged electrons scattered about in it like raisins in a pudding.