Maybe it's just something about what they are to begin with. I know we don't know anything at all about them, but you're right. They look just like creatures, and maybe it's simply their nature to eat light and life and don't think anything of doing it.

[No fancy magic, no foreign technology - perhaps they're just things that do what they were born to do. That's the theory he's going with anyway, until they get more information. It's both comforting and frightening to him. There's no evil intent behind any of it, but instead just... nothing. The stars are going out because of creatures mindlessly feeding on them the way cattle feed on grass.]