What approaches from the darker space beyond the landscape in the poem, The Thought-Fox?

The sky outside is dark and starless, but in an even darker space beyond something is approaching. The world in this space is a forest covered in snow; a fox is slowly and watchfully exploring it. As they approach, their paws leave dark prints in the white snow. Their shadow trails uncertainly behind them. As the fox nears the speaker, the world begins to fill with light.