what is the author trying to convey in "to fill a gap" by emily dickinson

There is actually an excellent analysis of this poem right here on this website:

"This poem is devoted to fathoming nullity or immensity as a spiritual quality. The “Gap” can be read to be a lack of faith, or a specific doubt that prevents one from having true faith. In this reading, by recommending that the doubter fill the gap with “the Thing that caused it,” she is saying that you cannot just ignore these doubts, but must deal with them straight on. This intention seems emphasized in the poems brevity, which doesn’t allow room for avoiding or circling an issue, but must get to it directly."