Both science fiction and murder mystery. The science fiction is hard-ish science, not space opera or fantasy. The mystery is a legit mystery that plays fair within the genre. A slight knowledge of physics and of the dual wave/particle nature of light is helpful going in.

It's a bit confusing at times, due to the consistent first-person narrative, and if I say anything more about that it would be a spoiler.

A few big plot points remain unsolved at the end - we never do find out why the mysterious beings are so angry at humans... (trust me, that's not a spoiler; it's way complicated enough that that's not giving anything away.)