In short:
Similar to what has "finally" happened to the horror movie genre, some have picked up actually scary elements to make horror movies not gore fests but actually scary subliminally, creepy, using the pauses and the sound of silence to create well crafted horror movies. This short falls into that category, similar (to give one example of another similar short) Night Swim or the longer movie Deep Dark (2015) which isn't a horror but has a similar (possible) use of the black hole.
End notes (spoilers) The subplot which could/oughta be introspection into your own history (making the person in the box be someone key in his history, or related to some child hood trauma, and the family'esque shadows would then be his previous family members who might have passed on - the light going out is then depression/suicide after loosing his last light in his life - the gf).
The black hole (the abyss) is something often related to in movies, and can symbolise everything from at large the struggle as a whole within, infinity or glue to hold together plots for those who aren't that careful about wha they put out.
End notes (spoilers) The subplot which could/oughta be introspection into your own history (making the person in the box be someone key in his history, or related to some child hood trauma, and the family'esque shadows would then be his previous family members who might have passed on - the light going out is then depression/suicide after loosing his last light in his life - the gf).
The black hole (the abyss) is something often related to in movies, and can symbolise everything from at large the struggle as a whole within, infinity or glue to hold together plots for those who aren't that careful about wha they put out.