Inaccessible Light?

Early in Eucharistic Prayer D in the Episcopal Book if Common Prayer, we find these curious words: “dwelling in light inaccessible.” I’m trying to recall where there is any other reference to this beautifully poetic representation of where God the Father dwells. And where is this light which we can’t access? It must be somewhere in the spiritual domain, since we’ve explored enough space to rule it out in the physically observable universe[s]. Then we have to wonder, given such a magnificent image, how we can call this exalted personage “Daddy,” as Jesus suggests. Trying to reconcile this mystery is more that my puny mind can manage. Can it all be true? How?