It's a well-written story and the feeling of dread is there, but to be honest, there's a lot I find confusing.

First, a nitpick: Observable universe refers to anything that can hypothetically be detected from Earth, whether or not we know about it, as opposed to things that can't ever be detected because the speed of light is limited (an astronomer would know this). Technology has nothing to do with where the "edge" is, and astronomers don't detect new objects based on distance, but capturing details of objects which are hard to detect.But in the end, it's not really relevant to the stars disappearing in the first place, so there's a long introduction with little payoff.

Then the narrator gives very few details about the actual catastrophe (despite their inquiring mind). I'm guessing the sky going dark means the point at which the Sun went out? Did draining energy mean just losing temperature, or some deeper energy which made everything alive? What happened to other people, exactly? Why is the narrator the last one left alive, and how do they even know this?

It feels like you put on a lot of weight on the beginning and the final part could use some more details. Otherwise it feels like a very generic "something evil happened and I'm the last one left alive" story.