In Hebrew and Christian Scriptures Angels are messengers of the Divine - there is plenty of discussion around as to whether they are actually embodiments of the Divine, or whether it's a way to describe 'visions' or dreams that people have, or whether they are real or symbolic. One thing that many of the accounts of Angelic encounters have in common is that they happen at night, many as part of dreams or visions.
The dark is a place where our imagination can roam - for better or worse. A place where we can live fearful of the shadows, or willing to explore the darkness. It can be the place where we live afraid of what we cannot see, and it can be the place where we see visions and dream dreams - a place where God may be able to speak to us in new ways and show us new possibilities.