The Last Star by Rick Yancey

The enemy is Other. The enemy is us.

They’re down here, they’re up there, they’re nowhere. They want the Earth, they want us to have it. They came to wipe us out, they came to save us.

But beneath these riddles lies one truth: Cassie has been betrayed. So has Ringer. Zombie. Nugget. And all 7.5 billion people who used to live on our planet. Betrayed first by the Others, and now by ourselves.

In these last days, Earth’s remaining survivors will need to decide what’s more important: saving themselves…or saving what makes us human.
 
- Goodreads Summary